Sending SMS messages requires a valid 10-digit mobile number (United States)
Symptoms
When sending an SMS message to a contact that has a 7-digit phone number stored, you may receive the following SMS message from a wireless carrier generated number:
"Error Invalid Number. Please re-send using a valid 10-digit mobile number or valid short code."
The intended recipient of the SMS text message will not receive the message that was attempted.
Products Affected
iPhone, iPhone 3G, (USA)
Resolution
This message occurs if the contact was synced from Windows or a Mac using only seven numbers to represent the contact's telephone number, or if the contact was entered manually on the iPhone using only 7-digits to represent the contact's phone number.
Update the contact to reflect the full number with area code. The contact can be updated by tapping Phone > Contacts > (Select Contact) > Edit and then change the corresponding phone number. You can also change the contact in your computer's address book and then sync iPhone.
In addition to updating the contact, you should delete any existing text conversations with that contact. The SMS conversations can be deleted by opening the Text application. If necessary, tap Messages to view the list of text conversations and then swipe left or right over any conversations with the updated contact and tap Delete.