Calling one Sipgate account from another?

Hello, can anyone shed any light on what may be happening?

I have two Sipgate numbers (both with credit on them).

The first (at home) is a Grandsteam BT101 which makes and receives
calls perfectly to PSTN and other VOIP services - and I can call my
other Sipgate number.

The other (at work) is a Sipura SPA3000 again working well receiving
and initiating calls to everything except one number - my first
Sipgate number.

Using the FXO port and the PSTN to VOIP gateway function of the
SPA3000, I can forward my work phone to any mobile or to any PSTN
number without any problem - but whenever I try to forward calls to my
Grandstream at home, it rings but as soon as I pick up, the call
fails.

I've tried emailing Sipgate support, but they did not comprehend me...

In brief: From Grandstream to Sipura all ok
From Sipura to Grandstream call fails on pick up


Any thoughts please....?

(PS. I've got all the ports forwarded correctly at each end, or else I
wouldn't be able to make and receive calls from other numbers)

--
Chris