Identify the hardware
Check the complete model and regional version, not just the familiar phone name. Supported radio bands, voice over 4G and SIM format can differ. A physical fit is not the same as compatibility with current calling services.
Identify the action
A new line, a replacement card, an upgrade and a number move use different processes. Keep the working service accessible while following the official instructions. Moving a SIM does not copy all photographs, applications or contacts.
Verify the result
After activation, test calling, messaging and internet access independently. Dual-SIM settings can send each function through a different line. Record which test fails before asking the provider for device-specific help.