Until now, you were limited to make only a single voice and video calls in Gmail at a time. Begining today July 20, Google has relaxed the restriction and now allow you to make or receive "multiple calls in Gmail".
This feature works across all call types (voice, video, and phone); the only restriction is that a maximum of two outgoing calls to physical phones can be placed at once.
"If you're in a call and make a second one, your first call will be put on hold while you talk on your new call. You can switch between calls by pressing the "Resume" button on the call you want to talk on, which will automatically put the previous call on hold," informs Google.
"Receiving incoming calls while you're in another call is just as easy: you'll receive a notification of the incoming call and can choose to accept it or not. If you take the new call, the previous call will be put on hold."
"Whether you make a second call or not, you can now put any call on hold -- useful in case you need to talk to someone in the room or grab something off the stove. Just press the "Hold" button and then "Resume" to start talking again," Google added.
[Source: Gmail blog]