I am using an amplifier / antenna system in my car and it does work well in fringe (weak signal) areas. However, it can NOT work as hand portable.
The standard cell phone transmitter puts out up to 500 mW (½ watt) of RF power close to your head, and that's considered acceptable. You can't buy one with more power. But you can buy a system of an amplifier, extrenal antenna, and a connector / adapter to hook your phone to the amp. Note I said EXTERNAL antenna. The amps push the output power to 3 watts (6x signal) but you cannot put that by your head, or inside a house. You mount the antenna outside in a good location whwere the signal is best anyway, and run a cable back to the amp. In a car, the high-power antenna MUST be outside the car on the roof so the roof panel shields the occupants from the RF field.
To connect phone to amp there are two classes of options. The simple way is you buy from the amp supplier a cable designed to go from your amp to your specific phone with its external antenna jack. It works well, but you have to be within cable distance of the amp. The more expensive alternative is, you buy a special amp and internal antenna system. This puts a low-power repeater antenna inside the house or car and any (even many at once) cell phone can use the system. Basically the phone and amp communicate over a low-power channel using an internal antenna in the house, and then the amp re-routes all traffic at high power through the external antenna.
Amps, antennas, etc are available from several companies. Mine is from Wilson, who seem to have a good reputation, and it works well. Be aware that there are several amp designs. Besides the option of direct cable connection vs. low-power relay with inside antenna, there is the matter of which frequency band. There is one band for analogue, and two for digital signals. The amp I use covers all three bands. There is also a question, sometimes, of which signalling protocol is in use, usually discussed in terms of which cell service provider you are using. The manufactureres / suppliers should be able to advise on all of this.
Now, this COULD be a way to give you reliable cell service in the house. But it still cannot meet your desire for portable reliable cell service, because ALL amplifier systems require significant power (could need big battery?) and cannot be used close to your body, to avoid possbile physiological damage from sustained exposure to high RF levels.
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