How can I contact my carrier

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

Or do you need the PUK code?, See this Apple article: Use a SIM PIN for your iPhone or iPad

Use a SIM PIN for your iPhone or iPad - Apple Support

Who is my carrier:

Go to -> Settings -> Mobile network -> scroll to Network selection you can see who your carrier is.

Surely you must know who is your cellular provider as you will get a bill from them every month. You need to contact them...

All carriers have a call center number. Google it