Can you help me find the least expensive local long-distance provider in San Francisco?

This is for our door buzzer system, which, for two of us in the building, calls our google voice numbers in the 510 area code. (We’re in 415 area code).

Here’s what we’re looking for:

* Cheapest plan
* Bills in partial minutes is a huge plus (most calls are 20 seconds at most)

We already have AT&T for the local line, we just need to add long-distance.