Hi, I bought a iPhone 6 full price from Sprint and I'd like to use it overseas for travel. The problem is, I can't activate the iPhone 6 on a european sim. I tried a Vodafone and KPN sim but both gave me the screen 'invalid sim'. I know it must be possible because I've seen other people activate their iPhone 6 full price from Sprint activate for overseas sim. Anyone an idea what I could've possibly done wrong?
You have done nothing wrong. What you have done is run into Sprint's draconian unlock policy.
Sprint requires you to have an active line for 90 days with your bill paid on time. Once you have done that you can call them and ask for an international unlock.
Sprint will only unlock internationally. They will never unlock your phone for domestic use.
iphonian, if you were a Sprint customer in good standing for 90 days with ANY phone before you activated the new iPhone 6, it can be unlocked right away. Unfortunately, you'll need to restore the phone from a backup after Sprint unlocks it for the unlock to take effect. Good luck.