asterisk users - Nov 2011 - 10-rc2: how to debug dropped calls?

I've been experiencing a number of dropped calls - both where I'm 
calling out and the call drops before answer, and where it's inbound and 
the call drops while I'm talking (usually at almost exactly 5 minutes).

I'm using dahdi 2.5.0.1 with a TDM400P connected to PSTN.

The console doesn't show anything ( dahdi 4 is PSTN):

     -- Native bridging DAHDI/1-1 and DAHDI/4-1
     -- Hanging up on 'DAHDI/4-1'
     -- Hungup 'DAHDI/4-1'
   == Spawn extension (DialOut, s, 16) exited non-zero on 'DAHDI/1-1'
     -- Hanging up on 'DAHDI/1-1'

And, of course, it's intermittent.

I don't see a way to set debug on for dahdi. How do you debug dropped calls?

sean