Problem with connection. Q2686H

HI! I have some devices with q2686h modules installed. They are rather old. The firmware version that has been installed in modules is: 661_09gg.Q2686H 1925236 100506 11:13. Then I’ve upgraded firmware and bootloader to version: R7.47.6.201212170952.Q2686H 2223904 121712 09:52. And in this version the problem with calls and SMS appears.
The answer to ATD+хххххххххххх; is “NO CARRIER” despite module is registered (+CREG: 1,1).
The answer to AT+CEER is error 254 (Call imposible).
BUT! Nevertheless sometimes I can make a call. 1 of 10-15 attempts is successful. So the problem is unstable.
The same situation with recieving calls. 1 of 10-15 is successfull. Devices with old software make and recieve calls without problems. So questions:

  1. Is there any difference between hardware in old modules q2686h and new modules (not refreshed)? (My hardware version is 4.03)
  2. Where can i find any changelog of hardware?
  3. Where can i find old version (661_09gg) of firmware and bootloader for it? Or maybe somebody has it and can share…
  4. Maybe someone has already faced with this problem and knows what to do… Help me, please!
    P.S. Sorry for my English :slight_smile: I’m from Belarus.