The issues you describe I too have had on and off with Gradwell VOIP.

I have days where VOIP is perfect - good quality sound and calls get through without issue.

I have other days with a terrible echo rendering speaking to customers a challenge. Not too sure if any of them can hear it their end.

Some days no calls routed - I think it's just a quiet day but in fact it's VOIP playing up.

So very variable. Generally it works but it's so frustrating when it doesnt. Hours spent trying to figure out if it's my side, Gradwells side, my ISP's fault...

I think either you just have to accept VOIP's quirks and live with them and yes maybe you'll lose the odd customer or 2.

Having said that BT doesnt have a good reputation and I'm sure Gradwell are better.