Timing

I have the following scenario

Rhino Ceros
Sangoma A200 card with 9 analogue lines and HWEC
Tb 2.4 updated to latest asterisk/zaptel/wanpipe

Since the system went in(and before the updates) we suffer bad quality on lines. Usually we hear the callers voice volume going down and up with some breakup.

It doesn't happen on every call but many.

I've looked at a lot of settings including gain/EC on off/updates/apic/cpi etc.

None really seem to make any difference. Also put in a managed switch, replaced the cabling on the phone lines.

I've read somewhere that putting a card in(like the a200) will mean it is used for timing.

I don't really understand what this means. But i would also like to know how does one tell if the card is in fact used for timing?

Any other suggestions would be welcome.

Thanks

R