the good old days of the brick mobile, you can change the aerial to a high gain one but these days, most are hidden inside. so that's out of the water. be careful about those stuff you stick at the back of your mobile that "claims" to improve reception. if they do, they will also increase the SAR!!!
now, have you tried different mobile for with the same provider and location? some mobiles are better than others. have you also tried another provider if the reception are better?
we have a signal booster for mobile reception but it's for commercial use. it's a big mask and can probably do KFC within a short distance of the mask! i havent seen any so far for consumer or home use.
you must be in the countryside not to get good reception.
