I have used one as overflow for 20 years call AllDayPA. The only thing they do which is annoying is NOT display their prices. I guess they want to have an opportunity to discuss. When they have provided the service it has always been good.
1. A company that's been around several years at least with a ecent reputation
2. Short or no contract
3. Fixed price per call, preferably no minimum monthly charge
4. The number belongs to you, not the answering company. If you leave, you don't lose the number.
5. No cost in transferring calls from your number to theirs and back again. (Although, if you want calling on a mobile, you'll need to pay for this somehow.)
6. The ability for the call handler to learn about your company over time so that the VA sounds like part of you company.
7. Access to call recordings so you can track wherther you calls are being handled correctly.