Why is there no guest contact information such as phone and email information in GDS bookings that are received for my property?

In some cases, travel agents will not provide guest contact details other than the name, since the guest has engaged the travel agent to act on their behalf.

The travel agent will in turn have their own processes and communication points to their client and must be the first point-of-contact for any hotel communications, as they are responsible for managing the guest's travel arrangements.
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Therefore, the travel agent is the person making a booking at the property and as with any person that makes a booking it's that travel booker that is the contact point for any communications.