Wideband repeater/amplifier?

Its just a really simple RF repeater.

You put some sort of antenna outside, and it collects all cell signal and basically retransmits it inside and vice versa.

We've had this stuff for years in Amateur Radio Land. Most good EE geeks can build one in an afternoon to work on the HAM bands. Cell phones are different because that frequency range is "restricted" and we're not supposed to build stuff that operates in that range or get our licenses yanked by the FCC.