best go phone coverage in scotland, where to buy?

We will be traveling all round scotland for a month, and we don't have a smart phone. We want to be able to contact B&B owners if we are lost or late. It seems that some companies are recommended for one area and some for others, but we will be going from Edinburgh to GlenCoe, to skye to Lewis and Harris, to Ullapool down to Inverness, and working our way back down to Oban and Mull. So which thing should we go with with the best chance of a phone call going through if we are lost? I'm really not even familiar with Go phones or the Scotland phone companies. We will arrive in Edinburgh, be there the next day and leave the next morning, so won't have long to find something.