I have a Samsung M540 with an unlimited texting plan. When writing a text message, it says I can use up to 1000 characters. But if I use more than 140 to 150+ characters (yes, it varies... for some reason) it attempts to connect to the internet in order to send the message (like when trying to send a photo) but I don't have an internet plan, so it fails to send. Why is it doing this?

Also, it's weird how it varies. Sometimes I can send a text with 140 characters and it will be fine, but other times it will try connecting to the internet and fail to send.