Support for Deutsche Telekom’s CompanyFlex SIP Trunks

Mixvoip’s Cloud PBX now supports Deutsche Telekom’s CompanyFlex SIP trunks, thanks to our Bring Your Own Carrier (BYOC) capability. This allows German business customers to use their existing CompanyFlex SIP trunks with Mixvoip’s phone system, maintaining full control over their telecom setup while benefiting from advanced PBX features. Interoperability has been successfully tested, ensuring reliable integration and smooth operation.

A challenge worth tackling

In 2020, Deutsche Telekom launched a new SIP trunking product called CompanyFlex. Unlike their previous solutions, CompanyFlex is built on an entirely different set of specifications—impacting both administrative processes and technical configurations.

When a client asked us to integrate CompanyFlex trunks into their setup, our team took on the challenge. Supporting this new standard wasn’t just about adjusting a few settings—it required rethinking how our system communicates with Deutsche Telekom’s network. Think of it as teaching your PBX to speak a new dialect.

We dug into the official 32-page specification document (1TR119), which outlines every detail needed to establish a compliant and reliable connection. This involved in-depth research, packet analysis with Wireshark, and code-level adjustments to ensure that all SIP messages were correctly formatted and aligned with CompanyFlex’s strict requirements.

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Key modifications for Telekom trunk registration

To fulfill the requirements for the registration of Telekom trunks, we implemented several key modifications:

DNS resolution

The outbound proxy isn’t resolved using a simple DNS A record. Instead, a DNS query first returns a NAPTR record, which then leads to SRV records that define the transport protocol (and possibly TLS encryption). This process ultimately provides multiple A records for redundancy.

Proxy identification:

An outbound proxy must be used for the CompanyFlex product to ensure proper connectivity

Client URI: The client_uri requires a different layout, and we have adapted it to be explicitly set using the SIP user.
Contact settings: The contact_user format required adjustments, so we adapted the retrieval process from the database. The SIP contact address is now determined based on either the authentication part or the entire user value.
Qualify: qualify_frequency and expiration timers are set to match their needs and ensure timely registration checks
Security mechanisms: The code enforces security by setting security_mechanisms to use SDES-SRTP and mediasec, and it specifies security_negotiation as mediasec.

These changes necessitated significant adaptations and extensive testing to ensure full compliance with Deutsche Telekom’s CompanyFlex trunks specifications and requirements.

Commitment to innovation

We continuously align with evolving telecommunications standards to keep our solutions reliable and future-ready.

If you are interested in acquiring your own integration with an external trunk, please do not hesitate to contact our team to get further information and arrange your own integration with our Voxbi PBX.