A voice call over a packet network is segmented into discrete call legs. A call leg is a logical connection between two voice gateways or between a gateway and an IP telephony device. Troubleshooting, debugging and sniffing should focus first on each leg independently and then on the VoIP call as a whole. You can isolate where a problem is occurring by determining which dial peer or call leg is having the problem.

You need to understand the call path through the router in order to determine where a problem is occurring.

• Call control application programming interface (CCAPI)—Three clients make use of the CCAPI: command-line interface (CLI), Simple Network Management Protocol (SNMP) agent and Session Application. The CCAPI main functions are:
– Identify the call legs (Which dial peer is it? Where did it come from?).
– Decide which session application takes the call (Who handles it?).
– Invoke the packet handler.
– Conference the call legs together.
– Start recording call statistics.
• Session application and dial plan mapper—The session application uses the dial plan mapper to
map a number to a dial peer (local POTS or remote VoIP). The dial plan mapper uses the dial peer
table to find active dial peers.
• Telephony and VoIP service provider interface (SPI)—The telephony SPI communicates with the
POTS dial peers. The VoIP SPI is the specific interface to the VoIP peers. Telephony or digital signal
processor (DSP) drivers deliver services to the Telephony SPI; the VoIP SPI relies on session
protocols.

Most used troubleshooting commands in Cisco (with sample IP, vrf, or number..etc)
# sh call history voice brief
# sh call active voice brief
# sh dial-peer voice sum
# sh ip ali
# sh ip int brief | i up
# sh arp

# sh cdp neigh

# sh call active voice compact

# sh call act voi id 2D60
# sh int des | include textqsqs

# sh run | b 50 vo
# sh run | b interface Serial4/0/0/3:1.1 point-to-point
# sh run interface fa2/0.19

# sh voice call sum
# sh voice call status

# sh voice port sum

# sh isdn stat
# sh isdn active
# sh isdn active serial 7/7:5
# sh isdn active Serial7/3:15 | include In
# sh isdn history

# sh controllers e1 7/4
# sh controllers e1 | in E1

# sh logging

# sh run int Gig 0/2

# sh ip route vrf unmanaged 0.0.0.0
# sh ip route vrf internet
# sh ip route vrf vrf1 10.100.200.224
# sh ip vrf int | inc 3/0:0

# ping vrf vrf1 10.100.200.170
# ping vrf vrf1 10.100.250.150 source 10.100.215.93

# sh processes cpu
# sh processes cpu history

# sh voip rtp connections

# sh dialplan number 03124440444
VOIP DEBUG Commands

Debug h245 event | asn1
Debug h225 event | asn1
Debug ras
Debug cch323 rtp
Debug ip tcp transaction

 

Sources ;

http://www.cisco.com/c/en/us/support/docs/voice/h323/14081-voip-debugcalls.html