1. betamax FUP is 150 min/per week. not 300 min/per week any more, since last summer. betamax did change without notice. Hence, you definitely run into FUP more frequently.
2. if it's a clone of betamax? you can check it here.
betamax price comparison
3. voxalot should work with fastvoip. but keep in mind, many posts mentioned new dial-plan wouldn't work in voxalot any more. at least, I still meet this problem. so I use simple dial plan only.
4. I am pap2 user. line#1 is voxalot. line#2 is betamax. so far so good. never run into FUP issue (unless it's real, i.e. over 150-min per 7 days).