Voxalot to Voxalot calls dropping out

[Oigle05]

This has got me stumped. Last week I opened up three free accounts with Vox and activated them in the normal way.
today I proceeded to connect up two of them to Minitar MVA11a adaptors.
They register ok, get dial tone ok and can call between the units ok. Problem is that the calls only last for about 45 seconds, then drop out.
I can call my old original, working number from either of these units and the call stays connected.
I can call the other units from my original...no problem.
If I put the settings for the other customer into my unit I get drop outs.
So it appears that the three new accounts have problems between each other only.
Can anyone tell me if they have experienced the same problem or not?
I think that maybe the three accounts might need to be re-set.
O