1. Charging you per-minute for use of the service.
2. Charging you a per-DID fee per day of use. Why? You are already paying for the service, and for the VOIP termination. BTW at Amazon scale, the DID numbers are either free or less than 10 cents/month per DID.
3. Charging you (in the USA at least) for the actual VOIP calling.
With 1) and 3), a USA call costs you 0.0048 cents per minute on inbound. Under usual circumstances, inbound calls to USA numbers are free.
They will also (should you request it) record the calls and store them in S3...