They can dial in directly from a telephone. 

They can use the dial pad on their Skype application if they have Skype's paid services. 

They can use the Direct Connect option via the email invite sent from the host. The host can send an invite from the Create form when scheduling the broadcast, or directly from the Studio. A high-speed internet connection, a computer, and headset are required to use this option. (Premium feature).

The host can use the Call a Guest option from the studio. Click the telephone icon, enter the guest telephone number (omit the ‘1’), and then click the ‘Dial’ button. You can select to place your guest in to the screening room, this must be done before clicking ‘Dial’. Note that the dial-out feature will only work for telephone numbers within the Continental US. (Premium feature).