[SR-Users] TM module timer question

Hello All, Situation, sip client on a smart phone, with a minimum registration time of 10 minutes, connected through wifi. Just after a reregister I walk out of range of the wifi connection. Then someone tries to call me, lookup location gives back that I am online and can be found at ip address w.x.y.z on port number xxxx, but I am not there because I walked out of the wifi range. Which timer do I need to use to get a failure route to reroute the call to a pstn gateway? I have tried the fr_timer which works, I set it to 5000, downside is that when I am in wifi range, I only have 5 seconds to answer the call. The fr_inv_timer is not the right one I think, as I will not get any message from the called number, as that device is not connected.. Link to a thread I missed is more than welcome. Thanks. Gertjan