gdgt

SOLUTION (suggested by fourblades):

I found a friend with a T-Mobile phone (a Razr, haha) and put my SIM card into that phone and turned it on, went to messages, and sure enough there were 23 messages in the inbox and 12 'quick notes.' There were even 5 messages in the outbox. All of the messages and drafts were either from TMobile at their 456 number, or from around the time when I got the phone. I deleted all of those, put the SIM back in the myTouch, and what do you know, the error message is gone and now I can access the SIM card message menu again.

It amazes me that none of this was available to me from the phone menu. It should have been, but that dialogue never actually opened. I have a feeling that I couldn't access the dialogue to delete messages from the SIM because it was full...which seems counterintuitive. It seems to me that you SHOULD be able to get in there when it's full in order to delete the messages. Maybe I'm just crazy.

I had this problem before! I forgot which window you have to go to. Another way to do this quickly till I find the solution is put your sim card to another phone and delete those SMS/text messages inside the SIM.

Oh from T-Mobile forums... go to your messaging icon and open your messages, then click on your menu (you should see to boxes on the bottom of your screen) compose and settings. tap on the settings (you will see a list of options) tap on the manage sims card messages then select the message(s) you want to remove by tapping on them (you will be given a choice of delete or save to phones memory) click delete this should help you clean up some of your messages, also, if your interested in saving your messages their is a third party application smsbackup (i believe is the name or backupsms) with will send your text messages to your google email account.

I know those SMS threads are stored in your phone and not your SIM so you'll have to delete the ones in your SIM card instead as instructed above. Hope that helps!