I've had about three pay as you go phones in the last 14 years - they are either gifts or are the cheapest I can get away with (although one had to be replaced because it was such a PITA to use). My current one cost me £30, but since its starting to have problems, my next one is going to be a smart phone. They start from £30 for a basic one, and since I now use my Ipod Touch for email, shopping and paying bills, there is no point not having one. The GPS is also handy.
You dont have to have a mobile, but it does make life a lot easier, and if you dont want anyone to hassle you, just switch it off. As far as bushcrafting is concerned, although being alone with nature is great, if that axe or knife slips, unless you have Lassie or Rin Tin Tin around to get help, a mobile is possibly the difference between some nasty looking stitches and bleeding to death unnoticed in a wood. Taking even a basic one is good sense, even if your not using it.