Please rephrase the question. I have no idea what you're talking about.
By "network provider", I assume you mean your cellular carrier, in which case I have no idea shat you mean by "network access code".
If I had to take a Wild ___ guess, I'd guess your phone is carrier locked and you want to use it on another carrier. Is that right? If so, see this document: How to unlock your iPhone for use with a different carrier - Apple Support