I just got my HTC Desire HD about 2 weeks ago and I ran into this problem about 5 days ago and have not been able to sort it out. My phone can text message anyone in my contacts and call of course but with my girlfriends number the first message goes through only. As soon as she replies to me and I try to reply back I get the 'attempted delivery many times' error message and the red triangle (I know, sounds like the perfect way out of talking to her, lol). If I delete that conversation and send her a brand new text it will go through but any texting to her in that same conversation box after that point will just error. It only happens to her number. I am on T-Mobile, I have tried un-syncing and re-syncing and I have deleted her number and made a new contact, no matter what it just will not work. I am going to go ask T-Mobile today but I have found others on the net with this issue but have not found a reply. I have not hard booted the phone yet though, not sure that will help any anyways. I do get a text message the next day though, it always sais 'you tried sending a text message to an invalid number, blah blah' and it goes on to say that valid UK numbers start with +44 or 07, but I have changed her number to both etc and still same problem. I even had her change her number on Facebook for the sync. Thanks for any help!!