Hi @Kaya4,
What happens when you try sending a premium SMS (if I understand you correctly, you are referring to that feature)? If you have previously blocked the feature (first time you use it you can choose to disable the feature) in your Xperia M5, then depending on your Android version you may activate it again. In order to do so in Android version 5 and earlier, go to Settings > Apps > Swipe to the All tab > Find and tap the Messaging app > find and tap Permissions > Premium SMS, then tap Always allow.
I am sorry to say that Google has not added management of premium SMS to Android version 6.0, so if it's blocked in this version, I recommend that you factory reset your device.
If it is not related to blocking the feature, as a first step I would advice getting in touch with your carrier for more information on the future on your carrier plan.
Let me know how it went! 
0online
1902743posts