My mobile is displaying "SIM REJECTED"

Did you happen to drop your cell? It may be a lose wire or it may not be inserted properly. Definitely try and investigate the problem with the provider or local store where the phone was sold to you.

I'm not sure. It could be a problem with either one. You should give more information such as your type of phone (mine is Samsung) and your service provider (mine is AT&T) so maybe someone can help you witha little mroe information. Have you tried calling your service provider? Most offer free customer service and they should be able to help. Or you can walk into a sales center and get a representative to help you figure out the problem. Good luck!

It could happen for any number of reasons... I read one of your responce about the phone being dropped... It could be a loose connection inside where your phone's not reading all the info off of your sim or reading your sim correctly... Or simply it could be the board inside the phone itself... Sometimes the phone itself may not be the issue... When the service provider has problems, it can't recognize your sim card in their system... That happens sometimes when you reboot or turn your phone off then back on... In your case, it's not likely but very, very very remote possibility...

Maybe try turning it off for a few minutes, then switching it on again. Try inserting another SIM and see if the problem continues with the new SIM. If it doesn't it means the problem is with the cellphone.