Dahdi interface remains busy

i have two pstn lines interfaced through a digium tdm22b interface card. i have set up an ivr for incoming calls from pstn with the WaitExten application. If a pstn user calls my system and enters a valid extension number then that extension is called. the problem is that if the call is diverted to the voicemail system and the pstn caller hangs up, asterisk still keeps on working by playing the files of the voicemail or waits for the voicemail timeout to occur and hence that DAHDI interface remains busy. i want that if the pstn caller hangs up, then asterisk should automatically know that and stop doing unnecessary actions. Kindly help me out.

What form of disconnect supervision does your PSTN provider supply?

What are the disconnect supervision options in chan_dahdi.conf set to?

i could not get the disconnect supervision information but the pstn company uses Siemens Hicom based exchanges. i tried various options like hangupondisconnect=yes, busydetect=yes, busycount=6, callprogress=yes but nothing could solve the problem.