I agree with the above - terrible reception. I have EE on my iphone 6 (had actually the same issues with an iphone 5 before) and I can stand in my office, directly by the window i.e. with the phone pretty much looking at the sky in the middle of the City of London and calls break every time.

This also happens in other areas of London for some strange reason. I am on a contract with them so not much I can do I guess. So be aware of this if you do choose EE.