I've been involved in the deployment of AT&T SIP Trunks for a couple customers recently and a few Gotcha's have been consistent between them.
911
First off, do not expect 911 calls to work immediately. AT&T SIP Trunks take up to 24 hours to populate all the necessary databases with the numbers and addresses. Ask the account team to verify that all the billing addresses associated with your SIP Trunk are correct. AT&T made a mistake in this area and caused one of my customers to not have 911 service for 5 days while it was sorted out.
Referred-by Header
This is absolutely required for certain call forward and call transfer scenarios and is fairly typical requirement for SIP Trunks in general. The details for this configuration are contained in KB Article 2500421.
Transcoding
Calls to certain numbers fail with a 488 Not Acceptable Here and in the diagnostic header there is a "cause=65". These calls are completely fine when you dial from another PSTN device such as a cell phone. Most of the time I've found that this is another AT&T SIP Trunk customer that has only one codec they can negotiate and you have a different codec. AT&T does not do any transcoding (which makes zero sense to me). In order for the calls to work you need to make sure you have at least G.729 and G.711 codecs in your allowed list and order them for your preference.
Enjoy...
