SAN FRANCISCO — Faced with withering criticism for its spotty iPhone service, AT&T blames in part a shortage of cellphone towers near homes and businesses. But it has a solution: put a miniature cell tower in your living room.

There’s a catch, though. You have to pay for it. And that is making some customers angry.

The size of a couple of decks of cards, these mini-towers act and look like Wi-Fi hot spots at cafes, and redirect cellphone calls from congested cell towers to home Web connections.

“It’s a fabulous idea, especially if you can’t get service, but to charge for it is insulting,” said Christina Zachariades, 28, of Manhattan who already pays $130 a month for iPhone service but cannot receive or make calls in her fifth-floor apartment on the Upper East Side. “How much more do I have to pay to get the service required for me to use my phone?”

Despite complaints like this, the technology is poised for big sales, thanks to price drops but also because of the entrance into the market by AT&T. Other companies — Verizon, for example — have already marketed their mini-towers for niche use to customers in places with limited cellphone signals, like basements or homes with particularly thick walls.