It's something to do with the latest iOS update.
Myself and two others I know get it regularly just recently.
I too am having the exact same issue. Very annoying does not depend on whether your using a different SIM card nor even if you've factory reset your phone.
seems to be entirely down to the 4G network as this never happens on WiFi.
looking on the EE forums there is a huge article on this over the last couple of months with the exact same issue as stated here but with no resolution in site other than use a different browser..... when using 4G