While mostly true, that's not totally true. It depends on what kind of phone you have.
Older 3G phones can be flashed from one carrier to another. Newer 4G phones such as the Sprint iPhone 6 can be unlocked to use Verizon and T-mobile, and there are still more strange combinations of unlocking such as Sprint S5 to any GSM carrier that are possible too.
There are many unlocking services, but they all depend on the original carrier of the phone, and the exact model of phone.
Contact me and I can help you unlock the phone for use with other carriers.