Source Connect is often touted as an ISDN alternative, and can be used to remotely monitor and send audio between locations. This allows for another studio from anywhere in the world to monitor or take control of the recording session and to directly communicate with the artist in the booth, ensuring that the takes that are captured are perfect.
This is usually the best option if you know you will have an audio engineer and a valid licence for Source Connect on your end.
Much simpler than Source Connect, Skype allows for the session to be monitored by anyone with a Skype account. This means that a director or producer can be in direct communication with both the artist in the booth and the engineer to give feedback on the artist's performance as it is delivered. Group calls are also viable so as multiple people can listen in on the session!
This is the most common way of remotely monitoring the session and is usually the best option if you are not monitoring the session from a professional recording studio.