Recently after running into problems with Three I decided to switch networks to EE, the prices were good (for my use) and the coverage some of the best in the UK.
I also switched my number, here is how to do it
- Make sure you have your PAC code from Three, if you don't give them a call and ask for it
- Once you have your new EE SIM, take out the Three one and replace it with the new one (you will have to accept you have a temporary number for now)
- Head over to here and look at the section Moving from another network
It's pretty much self explanatory from here.
Mine took 2 days to transfer over and I experienced no problems at all.
Note: If you use Messages on iOS, your temporary number will be added to iMessage, once your number has transferred correctly this will sort it self out.