If you get an error on Gmail while attemping to make a free call to an actual phone, that says this:
Call failed due to an error [Code: 3 Remote: true]
That error code (as far as I know), means that the person you are calling is on the phone already, or that Gmail is getting a busy signal while trying to call your buddy!
One of our readers (Wave Cool00) just commented that this error can also happen when trying to chat (audio or video) from the same PC but from different Gmail accounts. Thanks for the feedback!
If you have something to say to the contrary, let me know in the comments section!