When I try to send a text message to one of my contacts, the message doesn't go through. A notification pops up after a few seconds, "Message not sent", and a red triange with an exclamation point shows up next to the message.
When I select "message details", it says "Error Code: 21".
I have no problem messaging my other contacts.
I have been unable to find anything relating to this error code, and I'm wondering if anyone has any idea what it means?