Sipura 841 faint transmission

I've got a couple of SPA 841s, which I quite like. They came from the US so
I've had to make a few changes to get the right tones etc. One problem I
have is that the people I call tell me I'm very faint although I hear them
just fine. It's the same with both phones so I'm pretty sure it's a set-up
issue not a hardware fault.

Anyone had a similar problem?

TIA

Liam