1.) Can you send a sms to any other phone and it gets it?
2.) Is the phone your texting on the same network or a different?
3.) Are you mmsing or just smsing?
To be honest though its best to call your carrier and start tech support.
1.) If no other phones can get your text call in because either your missing a feature in their system or a blocking feature has been added to your account for whatever misc reason.
2.) If its on a different network then it could be a internetwork issue and needs to be reported otherwise will never be fixed.
3.) Some phones still can not receive Multimedia messages.