IPhone 7 Plus not taking calls when connected to BT Hub 6?

Very strange issue with the new BT Smart Hub (or BT Hub 6) and my Iphone 7 Plus.  I noticed that I was not able to make or receive calls, I have Hullomail installed and I was receiving notifications and e-mails for missed calls.

This is a strange issue I have had on and off for several weeks.  I tried to get support from BT but they were not much help.  I managed to narrow down the problem to the 2.4Ghz network, if I left my phone connected to the 5Ghz network I could send and receive calls fine.

I have another 2.4Ghz access point, I tried connecting with that one and it worked fine.  Still not sure what the issue is but it happens infrequently and I can connect to the 5Ghz network just fine.