Hi @Sandiford576,
Depending on your phone settings an MMS (Multimedia Messaging Service) can be more than just picture messages. This can also include attaching files other than images where supported, or "long messages". Texts only support 160 characters, then depending on your phone settings/software it'll either string multiple texts together, or it may convert to an MMS. Most often when I hear about this happening, it's long messages that are triggering this. Some phones may treat emojis as multiple characters too, using up the 160 much faster.
If you check your itemised bill you should be able to see when a message was sent, and to which number. This will hopefully help you locate the message in question. Check if your phone has a setting in the messages app to send long messages as SMS/text.
You also may have an option for iMessage/RCS/Chat messages, these will only be compatible when sending to a recipient who also uses the service, but iMessage is supported on all iPhones, and RCS/chat is available on most Androids for a few years now. These send your messages using data/Wi-Fi and stop images and attachments from being chargeable.
If all else fails, there's a number of 3rd party messaging apps which are widely used and available.
Hope this helps,
Jonathan
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