Funnily enough I was toying with this a few days ago.
What I found worked reliably with 2 way conversation for a Voxalot call to a registered Betamax client or to an IP phone logged on to the Betamax account is the following :
Set up an entry to match in the dial plan for the particular sip address (Eg. 4444)
Set up dial plan parameters in advanced mode:
Pattern: _4444
Replacement: Betamax_User_ID (Not a sip address, just the UserID)
Provider: Any Betamax Provider, unregistered is fine, in your provider list (No credit required)
Active: Yes, all others set to No
Save and test.
Dial 4444 from your Voxalot account and the call should go through to the Betamax client.
Hth .....