I cannot enter my phone number in the My Phone Number field. What can I do?

[AC Question]

I cannot enter my phone number in the "My Phone Number" field.

I go to Settings -> About Phone -> Status -> SIM Status and then I try to type in the "My Phone Number" field but I don't have access. I want to enter my phone number because I was told this would fix another problem I am having. When I receive phone calls only the phone number is displayed and not the name of the contact.

First of all, is this the solution to my problem? If not, what is. If it is, then how can I enter my phone number.

Thanks in advance for any advice you can offer.