HTC One M8 for Windows - Manual (Page 105)

What can I do during a call?
When a call is in progress, use the onscreen buttons to turn the speaker on,
put the call on hold, and more.
Turning the speaker on during a call
To decrease potential damage to your hearing, do not hold your
phone against your ear when the speaker is on.
On the call screen, tap speaker.
To turn the speaker off, just tap speaker again.
Switching between calls
If you’re already in a call and accept another call, you can choose to switch
between the two calls.
1. When you receive another call, tap answer to accept the second
call and put the first call on hold.
2. To switch between the calls, tap the bar at the top of the screen.
3. To end the current call and return to the other call, tap end call.
Phone calls 105