Some sponsors use backup processors and ask their affiliates to sign up for both.
Others have the affiliate sign up for the sponsor, and then the sponsor opens an account on behalf of the affiliate (on each of the processors).
Some others ask the affiliate which one they wanna use and do not give credit for the backup processor.
And then some just don't credit the affiliate for the backup signups, and there's no choice. If the affiliate asks, they say (something like) "phone signups only account for about 1% of signups."
Oh, and
http://www.probilling.com does the phone thing. So does Verotel if my memory serves me...