I can't turn on my phone - Apple iPhone 6 Plus (iOS8)
Cause 8 of 10: Your phone is locked to Vodafone
If your phone is locked to Vodafone and you try to use a SIM from another operator, the display will tell that your SIM is invalid.
Solution: To use a SIM from another operator, contact customer services to have your phone unlocked. Before unlocking a phone, certain requirements, which may induce costs, need to be met.