How can I stop text messages converting to mms on my i5s

[iMore Question]

BT keep charging me extra on top of my unlimited texts as they say I sent a card message or wrote in the subject area. I am sick of it and want to stop them being converted to mms please

 

[Just_Me_D]

Unless you the recipient(s) have iMessages enabled on their i-devices, any media sent via the Messages app will be sent as MMS.