I don't use VoipCheap myself but is Voxalot actually compatible with Betamax/VoipBuster as of May 2008? I knew there were big problems once, when Betamax banned Voxalot and/or denied free-calls status to calls placed via Voxalot. Has that been corrected yet? (I ask someone else watching this thread who knows the answer.)
As to your other question, why don't you try it and see? You should see it works just fine to call between accounts that way.