[Serusers] fr_invite_timer / voicemail

I may have posted this question before but I don't remember getting an answer so forgive me for posting this again. I am utilizing the fr_invite_timer so that I can forward a call to voicemail after about 20 seconds. The problem is that I would like to only do this for calls that are incoming to one of the registered users, so if the user is busy or unavailable we can send the call to voicemail. The problem that arises is that the timer is fired even for calls made by users calling the PSTN (via a cisco gateway). In those cases I want to let the call keep going without timing out since voicemail in that scenario is not relevant. Is there any way to differentiate the timer for these two scenarios? My methods for knowing whether the destination uri is a local registered user or the pstn is based on either is_uri_in() function or simply checking does_uri_exist(). The problem is that these are only known at "runtime" and the fr_invite_timer is set as a parameter when ser starts. Does anyone have any suggestions? Josh