I believe you can get increased range on some cell phones, but only passively. As its a transciever, which regularly changes frequency channel (and band) the on board transciever has an internal antennae switch, as well as RF amps (both tx annd rx) that constantly change their gain to extend battery life whilst maintaining a constant link with the nearest basestation. Any 'active circuitry' would have to sync with the tx/rx switch, and amps, which a few years ago were external, now they tend to be integrated all on one chip. (read: inaccessable singals).
It would require an external antenna, and modification of the phone, probably permenant making the phone less convenient for normal use. So, can it be done? I believe so, but with a LOT of hassle, and possible sacrifice of the phone itself. Is it practical? naaaa
For another 'impreactical' idea (one that takes time, effort and money) would be a secondary amateur radio link. Your phone left in your car, somewhere it gets reception, rigged via its hands free port, to a CB radio. You would have to add some way of detecting incoming calls to 'page' the CB radio, and a single data link to send control back to the phone end to 'answer' the phone, then just use the two way radio part.
I'm not suggesting its 'viable' but its an option
albeit a overly complicated one.