Auto SMS reply a missed call from a specific contact anytime I am looking for a solution to auto SMS reply a missed call from a specific contact anytime. All solution I found are either for all contacts, or for limited time intervals. Anyone has an idea?
As stated in the comments by Dan Hulme, what you ask can be achieved through automation.
Using Macrodroid:


Add a macro.
Select Call Missed on the Triggers section, then select the contact number.
On the Action sections, select Send SMS. Enter the phone number of your contact or select it from the contact list (you will need to give the app the appropriate permissions). Enter the message that should be automatically send to the contact.
No need to add a constraint. Give the macro a name, then save it. It will look like the following image:




Whenever you missed a call from the contact number you selected, an SMS will automatically be sent with the message entered.

Disclaimer: I am not related to Macrodroid developer(s).