Slide to open? what does this mean?

When your lock screen is activated, there's text at the bottom that says "slide to unlock" or "slide to open." When a call comes in when the phone is locked, a sighted user needs to swipe to answer if the phone is locked. I think you need to focus on the lower part of the screen and then double tap to select or swipe. If you have headphones, you can also answer by clicking the control button once.

If you do the magic tap with voice-over while a FaceTime call is active, it will end the FaceTime call.

When a call comes in, all I do is raise my phone, which activates the Lock Screen. Then I just do a two-finger double tap and the call is activated. To end the call, I do basically the same thing. I just pull the phone away from my face and the screen becomes active again, at which time, I do the two-finger double tap and the call is disconnected. This method is very reliable and shouldn't give you any issues.

When a call comes in, it should be noted that there are some buttons on the Lock Screen that you can utilize if you wish to. These include Accept, Decline, Message, and maybe a couple more that I've forgotton. Doing a one-finger double tap on the desired button will activate it.

HTH