[Serusers] Failure route weirdness....

[sip]

Okay... I'm a little stumped on what's going on here, so I'll start with a
general explanation before I bother posting my (likely hard to read) ser.cfg. 

I'm having a weird issue when calling from X-Lite to a SNOM 320 phone (I
haven't tried the SNOM 190 with a different firmware yet, but I don't see that
it would make much difference). 

I have a failure route which handles voicemail and such, and I have the invite
timer set to 27 seconds (roughly 4.5 standard US rings). Now, when I call from
X-Lite to X-Lite, or from Leadtek 8051 to X-Lite, or from SNOM to X-lite, or
from SNOM to SNOM, or from Leadtek to Leadtek, etc, etc... everything works
EXACTLY as it's supposed to. I have log messages telling me when the failure
route is being entered, and it hits it at right on 27 seconds. 

However, when I call TO the SNOM phone from any other device besides another
SNOM phone, I never hit the failure route until a 408 timeout at 58 seconds.
Now, I can't think of a reason for a UA to overwrite the invite timer on SER
or even how such overall weirdness would occur, but it's 58 seconds every time
like clockwork.. error 408. At that point, it enters the failure route and
does its processing (forwarding the call to voicemail as far as SER is
concerned but not as far as the UA is concerned... which just receives an
error message). 

Any ideas? 

Also... when I dial SNOM to SNOM, the SIP method I get in the ACC is PRACK (no
INVITE). Can someone fill me in on what a PRACK is? 

Is there something I should be looking at to figure out why calling one UA
behaves differently than other UAs? 

N.