That Gmail lets you call nearly any phone in the U.S. or Canada for free is pretty amazing and audacious. Now the embarrassment of bandwidth riches gets even richer, as you can make and receive multiple calls and put calls on hold. In other words, Gmail might be easier to use than your actual phone, and it's cheaper than Skype.
As detailed in a Gmail Blog post, Gmail now allows up to two simultaneous calls to be running inside its own browser space. If you're on your virtual Gmail phone and want to call somebody else to double-check something, you can open a new call window (either from the left-side call window or by quickly pressing "g," then "p") and pick a name or number. Your first call is place on hold, and you can come back to it by hitting the "Resume" button on its window, which also pauses your secondary call, if it's still open. If you get a ring while you're on your first call, you can choose to take that call and use the same pause/resume scheme to switch between your open calls.
Using this feature for calling phone numbers is convenient, but you can seemingly go beyond two calls if you're making a non-phone "voice" or video call through Gmail. My contact list doesn't contain more than two people I could call and immediately place on hold in the middle of a work day though, so I'll have to take Google at their word.
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Making phone calls in Gmail (technically through Google Voice) still requires a good Wi-Fi or cable connection, and likely a USB headset, unless you enjoy shouting and feedback. Still, if you find yourself somewhere with better web service than phone service, or if you don't mind scaling back on your cellphone minute usage, Gmail's quite the nice fallback option these days.
Kevin Purdy is a freelance writer, a former editor at Lifehacker.com, and the author of The Complete Android Guide.