[Serusers] invite timers

[Jeremy George]

   I need to set inv_timer to at least 30 seconds to account for the
long failover times apparently used by cellular providers.  Not hard
to understand; it takes a while to dig that cell phone out of the
pocket.

   But this seems to cause inbound calls to take the same long time to
failover to voicemail (an Asterisk system running on a separate box.)

   Do I understand the timers correctly?  Is there a way to cause a
relatively short inbound failover and keep a longish outbound failure?

- Jeremy