Spa-3000 quality problems in uk

hi all,
i am having some problems with call quality on a spa-3000. the spa is configured as a pstn gateway to my asterisk box (asterisk@home 2.5), whenever calls are made in or out of asterisk via the spa the quality is a nightmare. When the two people are talking at the same time the remote person (not on the asterisk box) keeps crackling and cutting out to the person connected to the asterisk box. However the remote user does not hear any of these problems, at the remote end the quality is perfect. I have checked the spa configuration and cannot find anything wrong, I have also checked the asterisk config and see no problems. The asterisk box also uses a voip outbound service from and the quality is perfect there so am not really sure what to check. I’ve tried the phone running on fail over by removing power from the spa-3000 but leaving all the cables in place and again everything is fine.

Also I have tried another spa unit and that does the same, both were running firmware 3.1.7(GWc).

Any info would be appreciated as this is a config which I could use over and again for clients and also the missus now refuses to use the home phone because of this.

many thanks in advance.

Rgds, Rob