To give an example of the cost,
Plivo[0]:
Make call: $0.0065/min
Recieve call: $0.0025/min
Twilio[1]
Make call: $0.0045/min
Recieve call: $0.0020/min
Flworoute[2]
Make call: $0.00833/min
Recieve call: $0.0050/min
So double to triple the cost from some providers for just this feature sounds very expensive. But perhaps with large commercial pricing this difference shrinks.
Yep. Surely the phone system costs for AT&T calling their own customers is so close to zero as to not matter even at telco scale, compared to the minimum wage they're paying the people on their end of the call.
(Also, isn't there some weird thing in the US where cell phone users get charged for _inbound_ calls? They might even make money on these...)
I expect that the hourly rate for the CSR dwarfs the cost of the telecommunications, and efficiently allocating resources and being more convenient for the customer vastly outweigh a few fractions of a cent per minute cost.
You missed the rates for receiving toll free calls, which are generally higher than both incoming and outgoing regular calls. For example, Flowroute is $0.00975/min.