I can't change my mtn smsc to +234803000000

Hi Ukpapi, you can check if you have the correct MTN message center number on your SIM. On your phone, visit your message settings, select SMS or Text Messages, then choose MTN...

Hi Utibe, you need to delete unwanted messages in your phone box in case if your inbox is filled up. You also need to check if you have the correct MTN message center number...