I have 500 minutes a month on my 3 mobile.I can call 01 02 and 07,mobiles,but I am not allowed to send texts?

This might sound a lot,but I hardly use it most months,as almost all numbers these days are 0800 or 0845,etc.I find most of the time its useless to me.I am highly penalised if I do send a text or use any other number.Does anyone know of a mobile phone company that will allow any number to be called,and if not,why not. Does BT have a mobile phone you could use like a landline,if not should someone invent one that does.