I just did a deployment where the PSTN was a Nortel and we used an AudioCodes 1000 gateway. Everything worked out well until we noticed that outside calls coming through the Nortel got dead air until the Lync user either answered the call or Lync sent the call to voice mail.
Seeing as how Lync to Lync calls behaved properly, and tracing showed the proper SIP 180 responses, the obvious culprit was either the gateway or the Nortel. Turns out that in an AudioCodes gateway there is a setting for allowing ringback to leave the system and head for the PSTN!
Now, I am not a gateway guru, but don’t you think that would be a desired thing to have? Well, it is off by default. Flip it over a bit and voila! Ringback to PSTN.