To call in to the meeting, you need the call-in number and meeting number or access code. You can find the call-in number and meeting number or access code in the meeting invite, if you received one. If you didn't get an invite, make sure the person who asked you to join the meeting gives you both numbers.
Call one of the numbers under Join by Phone in your invite or the number you were given.
When prompted, enter the meeting number and press #.
Enter your attendee ID, if you have one, or press # to be placed on hold in a virtual lobby.