Generally to know which debugs to use, you need to have an idea of what is wrong. This is because there so many components of both protocols. You also need to understand that in a typical PSTN connection using either h323 of mgcp, you have two seperate call legs.
1. Voip call leg (H323)
2. TDM call leg: E1/PRI/BRI etc
Now lets look at some of my fabourites debugs for looking at
VoIP Leg: (h323)
debug voip ccapi inout:
This is very useful in seeing the over all call flow. ie, which inbound dial-peer and outbound dial-peer are matched for the call. It also shows you any digit manipulations going on as well as codecs negotitated etc
debug h225 asn1 :
This is very crucial in looking at your h225 signalling. This is where we see call setup and call tear down.
debug h245 asn1
Now we are getting deeper here. To see capabilites negotitation we need to look at the h245 logs. This is where capabilities are exchanged, codecs, rtp ports, master/slave determination etc. So if you are looking at call drops/busy signal, you want to use this side by side with h225 asn1 debug
debug ip tcp transaction:
Often times people forget that there is a layer above the session layer where you have your h323/mgcp/sip protocol reside and that layer is called transport layer. I have seen cases where there were no h245 negotiation because of TCP connection issues..So this is where this one comes into its own.
Have a look at this document..it explains this very well.
https://supportforums.cisco.com/document/12142746/understanding-transport-layer-sip-and-h323
MGCP:
debug mgcp packets
This debug is the most popular of all mgcp debugs. However you need to understand how to inpterpret it for it to make sense.
debug mgcp events
On the ISDN/TDM side
debug isdn q931
debug isdn q921
show isdn status (you need this before you enen attempt to test anything.
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