FAQ/How can I setup snom3xx phones for TCP support

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Answer

The phone chooses the transport method according to this priorities:

1.If no DNS srv records are present the phone will use UDP by default. However, you can force the phone to use TCP by configuring the following setting:

Outbound Proxy: FQDN_or_IP_of_server;transport=tcp

Additionally there might be a need to change another setting:

Listen on SIP TCP port

which is for incoming connections and is only useful if the PBX does not use the same connection set up for the outgoing REGISTER. This happens in some cases but usually setting the "transport=tcp" parameter is sufficient because most PBXs do support persistent connections (TCP connection reuse).

2. If DNS srv records are present, e.g.

5 srv _sip._udp.pbx.test.de 5060 5060 pbx.test.de 5060 439833
4 srv _sip._tcp.pbx.test.de 5060 5060 pbx.test.de 5060 439832
2 srv _sips._tcp.pbx.test.de 5061 5061 pbx.test.de 5061 439832

--> TLS has highest priority, then TCP comes next. Otherwise UDP is used.

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