What's the matter with Samsung s5 answering the phone automatically?

Because you set up the call rejection SMS reply function.

If you want to cancel, I suggest you do this:

1. It's because you have set up the call rejection SMS reply function, just cancel it in the call settings.

2. According to your description, I suggest you try the following operations: Standby-Phone-Menu Key-Call Settings-Set Rejection Information Cancel.