As of April 1 2016 customers are charged 23.060% on prepaid transactions in San Francisco.
Prepaid MTS surcharge generally applies to amounts charged for:
Prepaid wireless airtime cards.
Prepaid wireless cards compatible with pay-as-you-go cell phones.
Prepaid wireless minutes.
Prepaid wireless plans.
Prepaid wireless refill or top-off cards.
Prepaid wireless 'e-Cards
And more....
Tax-free prepaid in California is going to end Jan 1, 2016. That's when a new law called AB-1717 goes into effect. It will shift California's state and local taxes on prepaid mobile service from being business taxes imposed on mobile operators to a new point of sale tax on end users. AB-1717 was sponsored by the CTIA.
For more information: http://www.boe.ca.gov/industry/prepaid_mts_surcharge.html