Missed calls SMS

Probably this is activated by your Carrier. iPhone does not have any settings to activate or deactivate it.

Unless you are choosing any of the options listed below when you receive a call (each time)

You can also swipe down on the call banner for more options.

Do any of the following:

• Tap Remind Me, then choose when you want a reminder to return the call.
• Tap Message, then choose a default reply or tap Custom.
• To create your own default replies, go to Settings  > Phone > Respond with Text, then tap any default message and replace it with your own text.

Note: In some countries or regions, declined calls are disconnected without being sent to voicemail.