isdn calls not working all the time

[peter2020]

Hi ,

we had swyx for many years , working fine all the time .We have a pri card with 14 channel. 

50 % of the incoming or outgoing call are failing , as we cannot hear the caller or viceversa. 

50% of the time when calling the isdn numbers , the caller cant hear the initial ringing tone.

 

Where the problem could be ? can you please help ?

 

Thanks

Peter