I don't use the call center module either (and am on Elastix 2.4). Do you know if the call center module has its own log, or have you tried tailing the asterisk logs?
does the used card determine the VOIP Trunk type ?
VoIP is not a trunk type. SIP and IAX2 are VoIP trunk types, but VoIP itself is not a thing. It's a generic reference to any voice over TCP/IP technology.