Thanks for your post on our Community Forums @nikkilouise, and a very warm welcome to you!
Sorry to hear of the issues you've been experiencing on the mobile services.
Can you please confirm if this happens in multiple locations, or just in one particular location?
Have you possibly tried placing the sim card in an alternative device, to see if the same thing happens?
I am also experiencing this issue, my number was switched to O2 last week and was working fine.
The last call I received was around 9am this morning but now people calling are getting the message "Calls to this number are not being connected"
Welcome to the community and thanks for taking the time to post here on the forums.
I’m sorry to hear of the issues that you’re having with your mobile at the moment, have you spoken directly to O2 about the issues that you're experiencing? If not, you can reach them using the contact methods here.