[Serusers] PSTN guard

I'm trying to setup PSTN gateway for termination and i would like to use SER as a guard + to collect CDR's. but my consurn is the BYE messages i don't know how many failed BYE messages i'll get in realtime operation because i can't get correct cdr's without BYE. Is there a way to use mediaproxy rtp timeout so when i don't receive BYE i the call will be terminated let's say in 30 sec. What kind of message does mediaproxy send after that idle time CANCEL? And what is better way to do billing? Set cron to check balance on 5 min. or to use exec module and do mysql query after every BYE or CANCEL message? Using cron will make me lose maximum of 5 min. * number of PSTN lines and with query after every call i will lose only charge for active calls. Regards