HTC Desire HD not sending text messages to certain people? (Vodafone)?

I have a HTC Desire HD, Android asked me to do a system update on the phone on Friday which I did. Since then I have only been able to text certain people. Other people I just can't send messages to their numbers, I have deleted them from my phonebook and added them back in again which still doesn't work. I rang Vodafone who say there are no blocks on my phone, they just offered to send me out a new simcard but I don't understand how it lets me send texts to some people but not to others? Anyone know what I could try?