Samsung galaxy S6 User Manual page 68

The ringtone or vibration will stop and the call will be sent directly to voicemail.
Reject a Call and Send a Text Message
You can reject an incoming call and automatically send a text message to the caller.
1. When a call arrives, slide Reject call with message up from the bottom of the screen.
The ringtone or vibration will stop and you will see a list of preset text messages.
2. Tap one of the messages to send it to the caller.
The selected message will be delivered to the caller.
Phone App 57