Can't remember the cost of calls but I think the service charge was around £90pm. It was something in the region of 50p per connected call I think plus the cost of the call from the base to the receiving end. I had it until 1981 when I moved to radiopaging and then onto my first mobile phone - £2200 (I remember it well!) (and that was a good price!) with call costs of 60p per minute with the first minute being charged for after the first 5 seconds!
Now I have a free phone and pay £35pm inc all the calls I want! And as for radio pagers... I haven't seen one of those for years!