No audio on second VSP line > help please

[Corbu']

I have this current scenario and I can’t find the cause:

Line 1 > I have a DID registered at Voxalot that is forwarded to a Canadian PSTN number via a Betamax clone (Jumblo). Let’s say I am currently on the phone so this route is in use (and so is a Line with Betamax).

Line 2 > At the same time another family member tries to access Voxalot from a regular phone via SipBroker numbers * and calls an overseas mobile phone also via this Betamax clone. The phone rings, the person picks up the phone but the line is total silence, no one can hear anything. If I hang up the call on Line 1 then try again for Line 2, then everything is back to normal, they can hear each other.

Also the same thing happens if I reverse the Line 1 / 2 scenarios. The first call is OK, but the second Line has no audio.

I don’t understand why this is happening. I have used Betamax clones before with two simultaneous calls going thru the same account and everything was OK. I am thinking it has something to do with PBXes Callthru but what and how can I fix it?
Any help in cracking this “bug” is greatly appreciated.

I don’t use an ATA at all (except X-Lite for occasional testing and a Nokia E51 for SIP/VoIP calling over WiFi), I use Voxalot for DID forwarding and I access my account from PSTN numbers using CallThru.

*with the help of PBXes Callthru; Voxalot is registered as a trunk at PBXes and is the only outbound route