What happens when attempting to dial over a channel that is "stuck"
Sep 29 15:46:12 VERBOSE[20131] logger.c:     -- Executing Dial("IAX2/101-1", "Zap/70/4507619973") in new stack
Sep 29 15:46:12 NOTICE[20131] app_dial.c: Unable to create channel of type 'Zap' (cause 0 - Unknown)
Sep 29 15:46:12 VERBOSE[20131] logger.c:   == Everyone is busy/congested at this time (1:0/0/1) 

A channel that fails looks like this:
Channel: 4
File Descriptor: 16
Span: 1
Extension:
Dialing: no
Context: from-pstn
Caller ID:
Calling TON: 0
Caller ID name:
Destroy: 0
InAlarm: 0
Signalling Type: PRI Signalling
Radio: 0
Owner: <None>
Real: <None>
Callwait: <None>
Threeway: <None>
Confno: -1
Propagated Conference: -1
Real in conference: 0
DSP: no
Relax DTMF: no
Dialing/CallwaitCAS: 0/0
Default law: ulaw
Fax Handled: no
Pulse phone: no
Echo Cancellation: 128 taps unless TDM bridged, currently OFF
PRI Flags: Call
PRI Logical Span: Implicit
Hookstate (FXS only): Onhook 

One that works looks like that:

hannel: 20
File Descriptor: 32
Span: 1
Extension:
Dialing: no
Context: from-pstn
Caller ID:
Calling TON: 0
Caller ID name:
Destroy: 0
InAlarm: 0
Signalling Type: PRI Signalling
Radio: 0
Owner: <None>
Real: <None>
Callwait: <None>
Threeway: <None>
Confno: -1
Propagated Conference: -1
Real in conference: 0
DSP: no
Relax DTMF: no
Dialing/CallwaitCAS: 0/0
Default law: ulaw
Fax Handled: no
Pulse phone: no
Echo Cancellation: 128 taps unless TDM bridged, currently OFF
PRI Flags:
PRI Logical Span: Implicit
Hookstate (FXS only): Onhook