Also there are so called "passive repeaters" which most users tha bought them say are just a scam.
Also make sure that the band is supported, 850 MHz UMTS/HSDPA is not the same as 850 MHz GSM. Your repeater (if bought in the USA) should specifically say it supports AT&T 3G service.
If the company in question was Orange, it would be 900 MHz UMTS/HSDPA & 1900 MHz GSM, repeater in the U.S. should then be T-Mobile compatible.
Also one note ... the repeaters are small, mini transmitters, As per FCC regulations repeaters are
perse interference with carrier cellular network. There is only one or two authorized manufacturers by the FCC to produce these home repeaters.
I was myself looking into a similar solution but decided couple of hundred dollars was not worth it to add some signal into my part-time home office that has (but not good one) reception.
This was a Chinese one and the cheapest I found on the market.
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