Grandstream 486 and voip.co.uk

I am setting up my old Grandstream 486 for a work colleague to use with
voip.co.uk.

Can anyone advise which settings from the following list to select for
each of the 7 preferred vocoders:

g.723.1
g.729A/B
pcmu
pcma
iLBC
g.726-32

I have tried various settings but the incoming speech always starts to
break up after about a minute or so. I have no problems at all with
voip.co.uk through my Linksys 2102.

In the grandstream settings I have put proxy.voip.co.uk against the sip
server and registrar.voip.co.uk against the outbound proxy. I have
disabled STUN as per voip.co.uk's instruction.

Anyone help?

Thanks.