On some phones it it can be easy to send a PXT by mistake and unfortunately i too have fallen victim to this.
PXT is also known as MMS (Multimedia Messaging Service) and TXT is also known as SMS, it can be easy to confuse the 2 when creating a new message to send to someone.
Also another thing to note is if you send a SMS/TXT that is longer than 480 characters, it gets reverted automatically to a MMS and this is especially common with android phones. I am unsure if this effects iPhone users.
(I am not an employee of Vodafone New Zealand and i am not associated to the social media team. I am a regular Vodafone New Zealand customer that volunteers my time here to help make Vodafone a better place for all)