Left hand speakers stopped after phone call

[Dave.Lee]

Hi people new to the forum and Audi, I've got an odd issue with my stereo. It's a 2011 Avant 2.0 tdi auto. When I got it in December no left hand speakers worked. Its a Concert head unit, I have removed this and tried a speaker on the outputs, all good, checked at the amp, all good. Bought a second hand Amp, plugged it in and all speakers worked spot on, done the balance and fader to confirm. Now the odd bit, all working great, made a phone call, ended call and no left hand speakers again. Is this connected? Tried both amps, reset etc, so annoying. Please can you help, 

Cheers, Dave

 

[Dave.Lee]

I've tried another amp, all the speakers worked spot on until I started up, then lost left side again. Does it need to be paired, if so can I do it?