The short answer
A phone can make basic calls yet still lack the voice, emergency, or network certification required by a modern carrier.
Step by step
Find the exact model and IMEI rather than relying on the phone's marketing name.
Run the carrier's current compatibility checker and confirm support for voice over LTE or the carrier's required voice technology.
Install available software and carrier-settings updates, restart, and retest calls, texts, data, and emergency-address settings.
If the phone is unsupported, compare replacement devices and plans without assuming the most expensive upgrade is necessary.
Back up authenticator access and personal data before moving to another phone.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.