• Thanks @james.carson Agreed.
  • Thanks @Bruce_Briggs. That's exactly what we were doing. As it happens, we have just discovered the problem though. Changing the DNS servers has fixed the problem, from (ironically) BT's DNS servers to Google's DNS servers. The phones now work!
  • Hi @james_carson Thank you for your suggestion. No, there are no Denies in the Traffic log for the phone IP addresses. Nor are there any Allows at the moment, unless I run a Ping from a Phone, which at least demonstrates it is able to reach the Internet.