Lync servers running cumulative update 4 (CU4) expose a slighly different timing behavior on outgoing PSTN calls. If call setup to the destination takes a bit longer and the PSTN gateway does not send any provisional reponse except “100 Trying” the call will be cancelled after about 10 seconds. It seems that the INVITE transaction timeout timer has been shortened in CU4.
With qualified Enhanced Gateways this is no problem since these products must be able to establish early dialogs by sending “183 Session Progress” including SDP. After receiving this response the timeout does not fire. This early dialog also ensures that all RTP endpoints are established before the call actually reaches the final destination (= early media) to avoid audio clipping at the beginning of the conversation.
If you run into this problem please check your gateway configuration and activate the appropriate setting. In OfficeMaster Gate products from Ferrari electronic AG this setting is called “Early Progress” and can be found under “VoIP-Parameters”.