@YvetaElectraDee sounds more like it’s failing to activate. is it actually activating successfully ? Or when you go to Settings > Messages under iMessage does it say waiting to activate or failed to activate.
It doesn't say anything because those useless functions are not active on my phone, not using them as there are way better apps. My iPhone sent 6 messages to UK alone yesterday. Never had such expenses with any other provider...
I've detected another strange thing with Vodafone. I sent a TXT on Wednesday 26/05 with probably 300 characters and Vodafone charged me 6 text messages. I thought it should count like 2.
Have you turned off FaceTime also as FaceTime also sends a SMS to UK to validate your phone number ?
Regarding the SMS, that is to do with with whether it is sent as GSM or Unicode format. If a single Unicode character is added the message is sent in Unicode and that reduces the character limit per message to around 70 (from around 160) and this is standard with any carrier.