Q: iPhone unlock

Hey,

today I requested to have my iPhone unlocked by my network and it is going to take up to 10 days. I have been notified that when I use another sim my phone will lock to that network and I will require to get the network to unlock it again. Is this true? I thought getting it officialy unlocked by Apple would permantely unlock it. I only needed my network to unlock it as I'm travelling to the UK for 2 weeks and when I come home I'm going to be using my normal network sim card again.

Thanks,

Jordan,