You can get an 0845 number free (telecoms world, 0870 042 1234) are just one provider.
This stops people thinking "no, they're not local"
Feature in your advert the area covered (either list towns by bullet point or show a map)
Get several 0845 numbers and use a different one on each advert, all divert to your phone, but you can get call response data from the phone provider (mine is emailed monthly) so you can see how many calls were made from which number. Then you have a clearer idea of which adverts are working best.
Alternatively, buy several pre pay mobiles and advertise your phone number thus........
Head Office 01xxx xxxxxx
Anytown 07xxx xxxxxx
Othertown 07xxx xxxxxx
Next town 07xxx xxxxxx
Toytown 07xxx xxxxxx
And so on. This gives the customer the impression that they are calling the member of staff nearest to them. Put a sticker on the phones so you know which one is which advert.
You could always do this format with several 0845 numbers.......
By the way, on a completely different subject, if you have ever sold a vehicle through Autotrader or similar and get fed up with loads of calls after it's been sold, why not get a cheap prepay mobile for this advert only, and switch the damn thing off after the vehicle is sold!