As this is an intermittent issue, its going to be hard to work out the cause of the issue. It could be issues with the local mast deliverying them or a problem with the sim.
First, check the EE/BT network checker and see that you have no network issues in the area, once you have done that have a chat with the online chat team. It maybe a case of them doing some trouble shooting or doing a sim swap.
If you are unable to resolve the issue after speaking with them pop back and we can maybe get a moderator to look at it for you..