Configure the router for aaa-new model
and do accounting for network and start-only or start-stop.
When ever you dial the isp and run ppp, the information would
be sent to the radius server after the ppp session terminates or
as soon as it start depending on your configuration.
This can then be parsed on the radius server
for the number of calls made.
aaa new-model
aaa authentication login default enable none
aaa accounting network isdn start-stop group radius