[carlosalbffgomes]

That's easy. When you call through Voxalot, Betamax knows that your call is bypassing through Voxalot, despite Voxalot in sending your IP address in SIP headers. That happens because the IP from which comes the first SIP invite message is from Voxalot servers. So, Betamax has two diferent IP address: yours (on SIP headers) and one from Voxalot's server (on IP headers). Differently, when you call directly from your computer or ATA, Betamax has only one IP address (in fact, they have both but they are the same).
So, what maybe happens is that Betamax no more honours the IP address sent in SIP headers from Voxalot servers to them. And Voxalot can not modify their IP address on IP headers because that is something that depends directly on the internet infrastructure and IP protocol. Also, don't forget that Voxalot servers are not at Voxalot facilities but they are rent from data centres in Holland, United States, and Australia. That means: the only way to overcome this FUP issue is if Betamax servers honour the IP sent to them in the SIP header and not the IP from which the first SIP invite message is coming from. Voxalot can not modify something that deals with internet infrastructure and in equipment rent on data centres around the world. It's easier to implement some kind of other solution at the users side, as suggested some where else in this forum.
Regards,