Are you currently with EE for your phone?
Not sure which plan you have selected, but it sounds like it includes the data/cellular add-on for the Apple Watch.
Basically the data plan I've mentioned allows you to connect the Apple Watch to the EE network if for example you were out of bluetooth range of your iPhone or for example if you wanted to go on a run or simply leave your iPhone at home, but still have the ability to be contacted if someone were to call you/or if you wanted to call them.
The Apple Watch on the data plan has its own number (for billing purposes if I'm not mistaken) but the Apple Watch doesn't use this number for anything other than billing, and it uses the same phone number as your iPhone.
So for example, in the example I've mentioned above, if someone called the telephone number for your iPhone, you Apple Watch would ring.
Not sure why you are being asked for a PAC code, but hopefully someone with more knowledge in that area will reply to your post and answer that for you.
Apologies for the long reply, and I hope it's helped clear things up for you a little 😀.
