Does an iPhone need a SIM card installed to be unlocked?

[Steve vRS]

My wife’s old EE locked iPhone 5S is being passed down the food chain to my son. She has taken out the SIM card and installed it in her new phone. However, we need to unlock the old phone as it is on EE and my son is using O2. Does the phone need to be connected to the network to be unlocked?

Will it have to be EE that unlock it or can O2 do it?

Thanks!