First of all, I'm far from an expert so bare with me if my questions seems ordinary :-)
We have 10 offices in Europe with a central CUCM 11 and local ISR 4321/4331 gateways running IOS XE 15.5 (or so I believe - an sh run returns version 15.5 in the beginning of the config).
We're in the process of migrating one of our offices in Germany from ISDN phone lines to a SIP trunk with Deutsche Telekom since Telekom is discontinuing ISDN as a product.
We have an external partner helping us and we've been relying much on the following document that describes how it should be done (or at least how it should be done back in 2017):
https://www.cisco.com/c/dam/en/us/solutions/collateral/enterprise/interoperability-portal/connecting-unified-communication-manager.pdf
It's definitely not been an easy experience and we're still not quite there.
First of all we ran into the problem that the document describes that we should use "registrar dns:sip-trunk.telekom.de expires 240 tcp auth-realm siptrunk.telekom.de" in the sip-ua section.
However sip-trunk.telekom.de doesn't have a DNS record and it should eow be reg.sip-trunk.telekom.de instead. The same is the case for the sip-server command.
But if I change the setup to use reg.sip-trunk.telekom.de instead, the DNS resolves fine but the REGISTER is rejected by Telekom because it then uses @Reg.sip-trunk.telekom.de as FROM domain in it's REGISTER request and that has to be @sip-trunk.telekom.de.
Currently I have a crude work around for this problem where I've created static host entries on the ISR gateway for sip.trunk-telekom.de and reg.sip-trunk.telekom.de so that it can be resolved by the gateway.
And now I'm able to make a correct REGISTER and have the SIP-trunk registered with Telekom and I can also receive calls.
However we cannot call out - when we send an INVITE we get a 403 response from Telekom.
They tell me that the problem is that I'm not using the same connection for my REGISTER and INVITE command and a network trace shows this to be correct (source port is different but the destination port is the same). This is not permitted.
However I already se conn-reuse in my "voice service voip"->"sip" config and I also use connection-reuse in my "sip-ua" config so from my limited knowledge, this should ensure that I use the same port.
What am I doing wrong?
It should be mentioned that Telekom only supports TCP for the requests.
I've attached most of the config to this post.
With regards,
Martin Moustgaard
