Because the geography is against you, I suspect, with the most likely problem the current placement of the towers (or rather where they were not allowed to be placed).
This probably means there is a 'gap' between these cells and when you move through this gap, you drop the call.
There is a similar spot on the N1 going over the Tygerberg hill where I sometimes drop a call. You get to know the spot down to a few meters. I've learned to finsh off calls as I approached Tygerberg. Saves a lot of swaering when the call drops.
So the local guys know all about it but they can't do much about it until they can get a better spot.
Geography is a huge thing in radio planning, especially in a place like Cape Town with it's complex coast line and the bloody great mountain in the middle, leaving precious little space to put up enough towers to serve everybody. And in many of those places people don't want a tower near their R60M home......
Then you have additional problems in mountainous areas. For example, if you're high enough up, not only will you be able to see a cell lower down, but you'll see all of them! Now your handset gets very confused and there goes the quality again.