Amazon already offers a calling feature on the Echo, however both parties on the call need to have either an Echo device or the Alexa smartphone app. Google Home is different in that you can call any home, cell or local business number in the US and Canada. Unlike Echo though, you won't be able to call another person's Google Home.
To call someone you'll need to have them saved as a contact, so you can just say "Hey Google, call Sarah". Google Assistant can already seek out details for your local takeaway, so you can ask it to find the number if you don't happen to have it saved.
Unless you're a Google Voice or Project Fi user, your number won't be displayed to the receiving called, which is a bit of a problem (there's a good chance they'll think you're a spammer) but Google says that it will make it so everyone can have their number displayed by the end of the year.
By the way, all of this works over Wi-Fi, so you won't need any type of cellular or other connection to make it work - or any paid plan. The feature will be rolling out in the next few days, but Google's not yet saying when we might see it arrive in other territories, like the UK.
We've been whitelisted to try the new feature early, and will let you know how we get on.