Can I use T-Mobile SIM card for EE network iPhone?

You have to unlock your iPhone before using it on a different network and the only way is to have the telephone network contact Apple and inform them that the phone is now unlocked.

Unfortunately EE seem to be very strict on unlocking iPhones. If you have purchased an iPhone or been given an iPhone which is locked to EE then it will be a lot easier for the original owner to ask for it to be unlocked. If this is not possible then EE may ask you to enter into an SIM contract for 6 months before they will unlock the iPhone. If you don't want to spend 6 months waiting and also the cost of a 6 month contract then you should look at Official iPhone Unlock.co.uk.They can unlock your EE iPhone in a matter of hours. The upfront cost is higher, but compared to a 6-month contract and the delay it may work out to be cheaper.

Unlocking means that you can use your phone with a different carrier. An unlocked phone is sometimes called a SIM-Free or contract-free phone.