When cow had me call all of Prosper's (415) numbers, I got an error at my local telco (ATT).
That's an indication that the call did not even get routed to the CO handling Prosper's CO... which could only happen if the telco at the other end fucked up a MAC, the trunks were pulled or disconnected for billing.
Otherwise if it was truly a problem by Prosper's PBX system, you'd get a fast busy, or an error message coming from Prosper's PBX or the CO in San Francisco. I'm getting an error from both ATT and Verizon Pennsylvania,