Virtual Toll Free with basic account?

[gowgowuk]

Hello,
Complete newbie here, so, please bear with me!
I've just set up my ATA (PAP2) and configure my VSP (voiptalk) to forward call to xxxxxx@voxalot.com, with a Voxalot free basic account and it all seem to work fine: I can make call, receive incoming calls and my voicemail is working fine as well. I tried the Virtual Toll Free button, as I'm quite keen to allow my family to call me for free, but whenever I try it using my local Virgin/Telewest telephone number (I typed +44131xxxxxxx), I get the reply:
"A call to "44131xxxxxxx" is not supported". So, i was wondering what could be the problem, and first of all, is this facility offered with the Basic account?
Many thanks for any help
Philippe