Blackjack II text question!?

I have an ATT Blackjack II. I absolutely love the phone. In the past few days when trying to send a text message to two different people, I immediately got an alert on my phone that says "message was not sent. try again?" Of course I tried again, and it went through immediately. I had a semi old SIM card in it, a few years old. So I got a new one last night to see if that helps. Has anyone experienced a similar issue with any phone or carrier? Does it sound like a network issue or a phone issue? Thanks!