I was in a similar situation a few of years back. Due to my hearing impairment, i used the old analog mobile phones ( the ones that looked and felt like bricks) The phones were cloned som many times and the bills were running upto ?500 a month. BT Cellnet always deducted the amount of calls that I did not make. it was going on for about 3 years.
Digital mobile phones entered the market and that made cloning difficult/impossible. BT Cellnet decide to upgrade the phone to a digital one. But as it was not compatible with my hearing aid, i did not get one. Was without a mobile for around a year, until a nice and friendly chap at Carphone Warehouse mentioned a INDUCTION LOOP made by nokia for hearing aids users. Ever since I have had the newest and upodated Nokia Mobile phone each year, provided it had the relevant induction loop coil to go with it. Am currently uing a Nokia 6310i and plan to upgrade to the 8910i in Xmas.