asterisk users - May 2011 - Dahdi channel stuck in "ringing" state

Hi,

For some time now I have noticed that our RBS T1 (asterisk 1.4.35, Dahdi 
2.3.0+2.3.0, TE410P) often has channels stuck in the state "Ringing",
like
this poor chap who got stuck on two calls in a row, apparently:

[excerpt from "core show channels"]

SIP/7157997-0000534b 7760308 at business:1   Ring    Dial(Dahdi/g0/7760308)
DAHDI/3-1            5130262 at from-pstn:1  Ringing AppDial((Outgoing Line))
SIP/7157997-00005347 5130262 at business:1   Ring    Dial(Dahdi/g0/5130262)
DAHDI/1-1            7761498 at from-pstn:1  Ringing AppDial((Outgoing Line))

If I look again a few hours from now, these entries will probably be gone, 
but they stick around for a very long time.  If I use "soft hangup" on
either channel both the SIP and Dahdi legs go away, and I see dialplan 
results from the call:

vigw3*CLI> soft hangup DAHDI/1-1
Requested Hangup on channel 'DAHDI/1-1'
     -- Hungup 'DAHDI/1-1'
   == Everyone is busy/congested at this time (1:0/0/1)
   == Auto fallthrough, channel 'SIP/7157969-0000532e' status is 
'CHANUNAVAIL'

I'd like to better understand how the call ends up in this state, and what 
makes it finally go away.  I'm assuming that in the meantime that channel 
on the T1 is unavailable, which is not a problem at the moment (plenty of 
free channels), but will surely be a problem as usage grows.

Any clues?

Thanks,

Jeff LaCoursiere
SunFone