You can use Google Duo (https://duo.google.com) from your web browser.

First you need to connect your phone number to email account with Google Duo using its mobile app (note: Recently the mobile phone app of Duo has been renamed to Meet).

You should be able to see all your contacts (assuming they are synced with the Google account) when you open Google Duo in the computer.

However, in this method, the recipient also needs to have google duo installed in their phone.

If you want to call phones which are not connected with google duo, you need to get a skype number and use Skype. But this is not a free service.