[Serusers] INVITE timers in/out

Hi, I have in my ser.cfg: modparam("tm","fr_inv_timer",40) set as a timer so that INVITEs coming from PSTN callers go to voicemail after 40 seconds. This works great for inbound called, however it seems to be a limitation for outbound calls. If the outbound INVITE doesn't receive an ACK in that time period, the call gets cancelled and drops to fast busy. Is there a way around this? Can I set different timers for inbound and outbound calls? 40 seconds seems to be a good amount time so that most outbound calls get answered by voicemail or actual people, but occastionally users will still get a fast busy. Any advice is greatly appreciated. Dan