Landline dead when hot weather - ongoing for 4 or 5 years

Every summer when we have the hot weather our landline randomly stops working normally mid afternoon to late evening. It only seems to happen on the really hot days and has happened for many years and I think I have reported it every year that it happens. By the time an engineer comes out it is normally working although there was one occurrence when an engineer did come out on a hot day when it was not working and it was confirmed that the fault was external to our property as the line was dead in the green cabinet.

The engineers don't want to investigate and suggest waiting until the change over to the new system that uses the VM hubs. We have not really pushed to get this problem resolved as lets face it, we only get a handful of really hot days however the past month with the current hot spell our phone has been playing up with it being dead nearly everyday from mid afternoon onwards.

Earlier in the year, I was contacted about switching over to the new phone system, but as we have two people over 75 that rely on the land line they won't switch us over. But, you can see the irony here is that the new phone system would be more reliable than our current land line especially over the past couple of weeks!

Can someone please get back to me. Either an engineer needs to investigate and work out where the real problem is or our transfer to the new phone system needs to be expedited.