'show isdn call-rate' on some platforms will show you if the call flow is going through a PRI.
Otherwise there isn't much in the CLI that will tell you this, and you would need VOIP monitoring software.
'show voice call stat', 'show voice call summary', and 'show call active voice brief' will allow you to see many active calls are up at any given time, but no historical data.
You may want to try switching from G.729b to G.729. G.729b is going to have a varying bandwidth since it only sends packets when there is speech. If you use G.729 you will have a constant-rate codec rather than variable and you will receive better voice quality and more reliable QoS/bandwidth requirements.
-nick