PSTN ring detection problem

I use trixbox CE , Asterisk with Sangoma A20101 ( 2FXO / 2FXS ports). The problem is when there is incoming calls from pstn line. The PSTN line is ringing 1s and pauses 9sec. ( period is 10sec.) When I call to asterisk’s PSTN line ,from other PSTN number or GSM mobile phone, I hear ring signal in my handset. Asterisk routes incoming call to local ext 11 and it begin ringings. After 10 sec. the ringing stops and phone displays “1 missed calls”. However, immediately after that the ringing is the renewed and after 10 sec the process is being repeated and phone displays “2 missed calls”, “3 missed calls” and etc. All the time the caller is hearing ring signal in phone handset.
I make another test. I build a hardware device, that is ringing 1s and pauses 4sec. ( period is 5sec.). I connect it to the FXO port on Sangoma card and everything works without any problems. No more any “missed calls”! I think the problem is 10sec. ringing period from PSTN line. How can I increase the detection period on incoming ringing of the Asterisk or any other method to resolve this problem?

The system:

  • operation system and kernel version - CentOS release 5.2 (Final), kernel-2.6.18-53.1.4.el5
  • asterisk version -
  • zaptel version - 1.4.12-1