Hello,
That's a good question.

We generate a new Bitcoin address for each new payment. Once we receive payment to a Bitcoin address we look it up in our database, deposit it into the applicable Link2VoIP account, and expire the Bitcoin address. Very simple.
We also recycle abandoned addresses after 72 hours in order to keep the Bitcoin database as small as possible.
Feel free to signup and try it out if you wish.
Cheers!