Sounds like SFO has a policy that nobody answers the phone.

Hence it would be true… the call center can’t call SFO, because nobody answers the phone. I have definitely encountered Avis counters that seem not to answer the phone. I remember repeatedly hitting redial for about an hour once, until someone at STS finally got sick of the phone ringing and answered. (At that point I was home, the car was in the lot (returned), but everything showed it as checked out still.)