Betamax providers do not accept url calls mostly.
But you can try this in Voxalot as wroten here:
http://forum.voxalot.com/voxalot-sup...html#post14445
But ok only you can reach then or caller need Voxalot account.
If you have premium you can maybe do something with call connection rules.
(sipbroker and enum point to your Voxalot and then forward to your sipdiscount)
(I have not tested enough yet)