For models with App support (e.g., Wave T1 mini & T5):
Please enable the "Speaker" function (or "External Speaker" mode) within the microphone's companion App. Once activated, the other person's voice from the call will play directly through your phone's built-in speaker.
For models without App support (e.g., WM620):
Please plug a pair of wired headphones into the 3.5mm monitoring jack on the microphone's receiver. You will then be able to hear the other person's voice in real-time through your headphones.