I see this a lot, usually a config problem. (sorry my Spanish is done by Google Translate)
1> !!Important!!
Agent Softphone or hardset MUST be registered.
If your phone is not successfully calling, it wont work.
Perform full two way test of the phones, make sure voice works both directions, make sure it does NOT hang up prematurely after 30 seconds.
2> NAT settings
PBX->PBX CONFIGURATION -> Asterisk SIP settings -> NAT
Make sure your external IP is there. Google "What is my ip"
If your SIP phones are connected using a DIFFERENT network than your ISSABEL server IP network, make sure those networks are listed.
So if issabel is 10.1.100.50 but softphones are 10.198.75.X network, make sure that network is also there. (example 10.198.75.0/255.255.255.0)
3>
A) AGENTS mode
PBX->queues = "STATIC AGENTS" -> each user= AXXX,0
"A" = Agent "XXXX"=extension " ,0 " = penalty. (Normally set to 0)
So if SIP extension 8001= A8001,0 inside the QUEUE configurations
B) AGENT CALLBACK.
PBX-> queues =DYNAMIC AGENTS - each users = SXXXX,0 XXXX=extension number.
"S" = SIP "XXXX"=extension " ,0 " = penalty. (Normally set to 0)
So if SIP extension 8001= S8001,0 inside the QUEUE configurations
4> INBOUND CALLS
Call Center-> Incoming Calls -> Queues
Make sure that your queue that your agent belongs to is listed here and is "ACTIVE"
Hope that this helps!