Stochastic one-way audio on FXO

Hi Folks,

I have no idea what to do with the following problem:

Scenario: FreePBX with SIP extensions, one OpenVox A800P card with 2 FXO card. At the other end of the analogue lines there is a Cisco Cable modem with 2 analogue ports. I have two phone analogue numbers.

The problem is the following: only 1-2 times per week the external caller have no audio (the extension doesn’t hear the caller), but the caller can hear the extension. No error in the FreePBX. When this error appears, the only solution is to restart the PBX PC, AND restart the cable modem too. If I restart only the PC, the error still remains.
I changed the modem - not solved the problem.
I changed the two FXO modul on the card - not solved
I changed the slot on the card - not solved

How could I determine, where is the problem?

Are the SIP extensions on the same network as the PBX? How does the cable modem fit into the issue?

This one-way audio issue appears on the FXO trunks only (outgoing and incoming calls). Between two extensions everything works fine.
The cable modem has two analogue phone connectors, the two FXO modules are connected to here.