I have a similar problem. From the asterisk console, I get:

channel.c:5681 __ast_request_and_dial: Unable to request channel SIP/474_5091

The message goes out as follows:

Exten: 474_566
CallerID: 5091 <474_5091>
Action: Originate
Async: 1
Priority: 1
Channel: SIP/474_5091
Context: from_local_siptel_nat
ActionID: 8

And is refused as follows:
Event: OriginateResponse
Privilege: call,all
ActionID: 8
Response: Failure
Channel: SIP/474_5091
Context: from_local_siptel_nat
Exten: 474_566
Reason: 0
Uniqueid: <null>
CallerIDNum: 474_5091
CallerIDName: 5091

I suspect, but do not know for sure, that it's the CallerIDNum being 474_5091 that's the problem, in asterisk rejects calls with non-numeric CallerIDNum in them. manger.conf is set up as follows:

[admin]
secret = XXXXXXXXXXXXXXXXXXXXX
deny=0.0.0.0/0.0.0.0
permit=127.0.0.1/255.255.255.0
read=system,call,log,agent,user,config,command,reporting,cdr,dialplan,originate,agi,cc,aoc,all
write=all,system,call,log,verbose,command,agent,user,originate,all

[SIP/474_5091]
type=extension
extension=474_5091
context=dept_1226_some_shared_context
label=5091

[SIP/474_566]
type=extension
extension=474_566
label=566
context=dept_1226_some_shared_context