If you have a carrier unlock iPhone, you’re free to go with the carrier of your choice.
If you have a carrier locked iPhone, see this Apple article: How to unlock your iPhone for use with a different carrier -> How to unlock your iPhone for use with a different carrier - Apple Support
- Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.
You need to contact your Carrier.
iPhone (4)
If you talking about an iPhone 4, > 13 year old - running iOS 7.1.2, you have an outdated iPhone, that may be the cause of the problem.