Have a look at I'm being charged for texts when I thought they were included in my plan
Typically, phones will convert text messages into picture messages when:
- you send a group text message
- your text has a non-standard smiley or symbol (such as an emoticon or emoji)
- your text goes over the character limit (on some phones this is 160 characters, but on others it's more)
- you add text to the subject field
On some phones you can clearly see when this happens - for example, you might see a character count changing to a file size.
Some phones let you switch this function off, or enable you to set up an alert when it happens. You'll need to check the manual or go to the support website for your phone to see to do.
