If you use emojis in a message it may be treated as an MMS depending on how long your message is and where you get the emoji from.
Each emoji is worth two characters, so if you send a message of 100 characters with an emoji on the end, this will be sent as an SMS and your normal SMS rate will apply. However, if you send a message with 159 characters with an emoji on the end, this will be 161 characters in total, and will be sent as MMS.
If you use the built in emoji feature on your device, messages with an emoji that are:
- 160 characters or fewer are sent as a text message.
- Over 160 characters are sent as an MMS.
If you use a downloadable app feature to send an emoji in a message, then this will always be sent as an MMS no matter how long or short the message and your usual MMS rate will apply.