Strange one way audio

[Tranworld]

My voxalot is 968032 and I have the following providers for dialling out

VoIPCheap
Pfingo

my E52 can logon to Voxalot and make call out but call through Pfingo only have one way in that I can't hear the other side but they can hear me. But if I call the same number using VoIPCheap then the audio is fine.
Now if I logon to pfingo directly then the audio is also fine, therefore only via Voxalot that the audio in pfingo only is disrupted, the question is where should I look ?

By the way don't blame the E52 since my E51 also displayed the same problem

Remember VoIPcheap works fine and pfingo also works fine on the same phone if used directly.