What do I do when the phone doesn't recognize my sim card?

when i put my sim card into my phone it says sim card not recognized even though the network is unlocked

Helps to know which device you are referring to and what OS it's currently on. Lots of folks might be able to help you, but without that info, kind of shooting in the dark.

Has the SIM card been working in that phone? Or is this a new to you phone? If so, are you sure it is unlocked and works with the bands that the SIM card works with?

Does the SIM card still work in your "other" phone?

Inspect the PINS in the SIM Tray....