Flexisip vs Kamailio vs Routr

Flexisip, Kamailio, and Routr are all free, self-hostable SIP servers, but they sit at different points on the maturity-versus-simplicity spectrum. Kamailio…

Flexisip

Open Source · From Free (AGPLv3 open source; commercial license available)

Best for: Teams that want a modular SIP suite with presence, push notifications and end-to-end encrypted messaging built in, especially those pairing it with Linphone clients or wanting an optional commercial license and vendor support.

Kamailio

Open Source · From Free

Best for: Carriers and VoIP service providers that need extreme, battle-tested throughput and are willing to invest in SIP expertise to configure a highly extensible routing engine.

Routr

Free and open source, self-hosted; no official paid SaaS tier for Routr itself · From Free (self-hosted)

Best for: Cloud-native teams who want to deploy a lightweight SIP proxy, registrar and location server on Docker or Kubernetes and manage it through a REST API, CLI or web GUI.

At a Glance

	Flexisip	Kamailio	Routr
Primary category	Communication	Communication	Communication
Rating	Not documented	Not documented	Not documented
Pricing model	Open Source	Open Source	Free and open source, self-hosted; no official paid SaaS tier for Routr itself
Starting price	Free (AGPLv3 open source; commercial license available)	Free	Free (self-hosted)
Free plan	Not documented	Not documented	Not documented
Free trial	Not documented	Not documented	Not documented
Platforms	Not documented	Not documented	Web
Team collaboration	Not documented	Not documented	Not documented
AI features	Not documented	Not documented	Not documented
Public API	Yes	Yes	Yes

Standout Differences

Flexisip bundles presence, push and encrypted messaging

Flexisip ships dedicated modules for RFC 8599 mobile push notifications, SIP SIMPLE presence, a B2BUA for media/identity bridging, and LIME-based end-to-end encrypted messaging, giving operators more built-in capability than a bare SIP proxy.

Flexisip

Kamailio has the longest track record at the largest scale

Dating back to 2001 as SIP Express Router, Kamailio is described by its maintainers as routing billions of VoIP minutes globally every month, with asynchronous TCP/UDP/SCTP transport built for thousands of call setups per second.

Kamailio

Routr is purpose-built for Docker and Kubernetes

Routr ships as a Docker image with Helm charts for Kubernetes, plus a REST API, CLI and web GUI, making it the most straightforward of the three to fit into a cloud-native, infrastructure-as-code deployment workflow.

Routr

Support models diverge sharply

Flexisip is the only one of the three with an optional paid commercial license and direct vendor support from Belledonne Communications. Kamailio explicitly has no official vendor support and relies on community or third-party consultants, while Routr has no documented managed or support tier at all.

Flexisip, Kamailio, Routr

Feature-by-Feature

Core SIP Signaling

Feature	Flexisip	Kamailio	Routr
SIP proxy and registrar routing	Available	Available	Available
Presence server (SIP SIMPLE/XCAP)	Available	Available	Not documented
Mobile push notifications (APNs/FCM via RFC 8599)	Available	Not documented	Not documented

Deployment and Scale

Feature	Flexisip	Kamailio	Routr
Docker/Kubernetes-native deployment tooling	Limited	Not documented	Available
Lightweight/embedded and IoT-capable footprint	Available	Not documented	Not documented
Multi-tenant, multi-domain isolation	Not documented	Not documented	Available

Management and Support

Feature	Flexisip	Kamailio	Routr
REST API, CLI or GUI administration	Limited	Available	Available
Official commercial license or vendor support	Available	Unavailable	Not documented

Pricing Compared

Starting price reflects the lowest paid tier, not the full cost for every team size or usage level.

Flexisip

Open Source (AGPLv3) — Free Free forever

Commercial License — Custom (contact sales) Custom

Kamailio

Open Source — Free n/a

Routr

Open Source (self-hosted) — Free n/a

Pros & Cons

Flexisip

Pros

Fully open source under AGPLv3, meaning the code is auditable and self-hostable at no license cost.
Battle-tested at scale, having powered the free linphone.org SIP service continuously since 2011.
Modular design lets operators deploy only the components they need, such as just the proxy and push modules.
Strong standards compliance (RFC 3261, RFC 8599) ensures interoperability with any standard SIP client or phone.

Cons

Requires solid Linux systems administration and telecom networking expertise to deploy and operate; it is not a plug-and-play consumer product.
Belledonne Communications does not offer an official managed cloud hosting service, so self-hosting or third-party hosting is required.
Documentation is technical and written for telecom engineers rather than general software developers.
Smaller community and ecosystem compared to larger open source telephony projects like Asterisk or FreeSWITCH.

Kamailio

Pros

Completely free and open source with no licensing cost
Proven at massive scale, routing billions of VoIP minutes monthly
Highly configurable and extensible for custom routing logic
Long operating history and mature, battle-tested codebase since 2001
Broad backend and protocol integration options

Cons

Steep learning curve requiring SIP and telecom protocol expertise
No official vendor support; relies on community or third-party consultants
Configuration syntax can be complex for newcomers
Requires self-hosting and infrastructure management
Best suited to large-scale deployments, overkill for very small setups

Routr

Pros

Free and open source with no licensing cost for the software
Cloud-native design that deploys cleanly on Docker and Kubernetes
Multiple control interfaces (CLI, REST API, GUI) suit different operational preferences
Extensible architecture for teams with custom SIP routing requirements
Built by Fonoster, a team focused on open-source communications infrastructure

Cons

Requires SIP and telephony infrastructure expertise to deploy and operate safely
No official managed/hosted SaaS version documented, so operations burden falls on the deploying team
Smaller community and ecosystem compared to established commercial SBC and SIP server vendors
Corporate details about the maintaining company are not consistently documented publicly

Use Cases

Choose Flexisip: Teams that want a modular SIP suite with presence, push notifications and end-to-end encrypted messaging built in, especially those pairing it with Linphone clients or wanting an optional commercial license and vendor support.

Choose Kamailio: Carriers and VoIP service providers that need extreme, battle-tested throughput and are willing to invest in SIP expertise to configure a highly extensible routing engine.

Choose Routr: Cloud-native teams who want to deploy a lightweight SIP proxy, registrar and location server on Docker or Kubernetes and manage it through a REST API, CLI or web GUI.

Flexisip

VoIP Service Providers and ITSPs — Internet telephony service providers use Flexisip as the core SIP proxy and registrar to operate a carrier-grade VoIP service for their subscribers.
Enterprise Unified Communications — Enterprises deploy Flexisip alongside Linphone clients to build private, self-hosted internal calling, messaging and conferencing infrastructure.
IoT and Embedded Communication Devices — Device manufacturers embed Flexisip's lightweight modules to add SIP-based calling and messaging into intercoms, industrial equipment and other connected hardware.

Kamailio

Carrier-grade VoIP routing — Telecom carriers deploy Kamailio as a high-throughput SIP proxy and router for call traffic.
WebRTC communication platforms — Developers build browser-based calling and messaging apps using Kamailio's WebSocket-enabled SIP signaling.
Enterprise unified communications — Enterprises build internal SIP-based voice and presence platforms using Kamailio as the signaling core.

Routr

Self-hosted VoIP carrier infrastructure — Telephony carriers and communication service providers deploy Routr as their SIP proxy and registrar to avoid the cost of commercial session border controllers.
Cloud-native SIP deployments — Engineering teams deploy Routr on Kubernetes or Docker to run scalable, containerized SIP infrastructure alongside other cloud-native services.
Custom call routing logic — Integrators extend Routr with custom Processors and Middleware to implement specialized routing rules for enterprise or multi-tenant SIP deployments.

Frequently Asked Questions

Which of Flexisip, Kamailio, and Routr is best for a large-scale carrier deployment?

Kamailio has the longest operating history in this comparison and is reported to route billions of VoIP minutes monthly, making it the most proven option for carrier-grade, extreme-throughput deployments. Flexisip has also run production infrastructure since 2011 via linphone.org, while Routr is the newest of the three with a smaller published track record.

Can any of these three run on Kubernetes?

Routr is explicitly built for Docker and Kubernetes, shipping with Helm charts for cluster deployment. Flexisip supports Docker-based and Nix-based reproducible builds but does not document Kubernetes-specific tooling, and Kamailio's containerization approach is not detailed in the provided data.

Do Flexisip, Kamailio, and Routr include mobile push notifications for iOS and Android?

Flexisip has a dedicated Push Notification module implementing RFC 8599 for waking mobile SIP clients via Apple's APNs and Google Firebase. Push notification support is not documented for Kamailio or Routr in their published feature sets.

Is commercial support available for any of these SIP servers?

Flexisip is the only one of the three offering an official paid commercial license with direct vendor support from Belledonne Communications. Kamailio's maintainers explicitly note there is no official vendor support, relying instead on community or third-party consultants, and Routr has no documented managed support tier.

Which tool has built-in presence and instant messaging features?

Both Flexisip and Kamailio document dedicated presence server capability using SIP SIMPLE and XCAP standards, with Flexisip adding LIME-based end-to-end encrypted messaging. Routr's documented feature set covers proxy, registrar, and location server roles but does not mention presence or messaging.

Do any of these three offer a hosted or managed SaaS version?

No. All three are self-hosted, open-source SIP servers with no official managed SaaS tier from their maintainers. Flexisip's commercial license covers licensing and support, not hosting; Kamailio and Routr both require the deploying organization to run and operate their own infrastructure.

Read the full Flexisip review · Read the full Kamailio review · Read the full Routr review

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