iPhone not working

[Ir12345]

I bought a phone from you and now it is not working for 4G and calls only work sometimes 

[pgn]

Does the problem happen everywhere, @Ir12345 - or just in a specific place?

Does the problem move with the SIM if you swap SIMs with another O2 phone?

Have you checked to see if any work by O2 on the mast nearest you is happening?

Guide: How can I sort out my network issues? has the links you need, to the O2 Network Status Checker.

If you have good broadband WiFi where you are, this may help:

https://www.o2.co.uk/connectivity/wifi-and-4g-calling

[Oxonian]

This is also worth looking at @Ir12345 :-

My Network App - Find Out More About Your Network Coverage | Apps | O2

https://www.o2.co.uk/apps/my-network