On the Yealink T19P you are able to make multiple calls by following the steps below.
While on an active call:
1. Press the Hold soft key to place the current active call on hold
2. Press the NewCall soft key
3. Enter the desired number for the second call using the keypad
4. Press , or press the SEND soft key
To switch between the calls, press or and then press the Resume soft key to retrieve the desired call.