Some places like in UK here allot free minutes each month. The scripts attached are to display the remaining minutes and the used minutes each month.

Steps:
1. Enable endtime recording in database following the triggers script here
2. Copy freecalls.sh and calltimerpopup.sh in /home/user/scripts and make them executable (chmod +x freecalls.sh calltimerpopup.sh).
3. Copy callhangup and freecalllimit to /etc/dbus-scripts.d
4. Edit freecalls.sh for the freeminutes, starting date and number of sms which count to 1 minute. Leave SMSPERMINUTE=0 if they are not counted in minutes.
5. If there any phone numbers which shouldnt be counted edit FREE line. For me numbers 123 and 333 are free.
6. Reboot device and make a gsm call. Popup will appear after the call is terminated.

*Dbus-scripts need to be installed.
*Thanks to DocScrutinizer for sorting out the niggles.