There are 3 possible reasons for this:
(note: in all cases, you can still Request GPs via the system)
1. The doctor has not published their contact details
2. You are using a FREE practice account (unable to see GP contact details)
3. Your practice is outside the doctors work radius but they have decided to show you their availability anyway (details below).
For example, a doctor has a work radius set in their account of 50 miles (this is how far they are willing to travel to work and affects the list of Practices they advertise their availability to)
Your practice is outside of this doctors Work Radius so, normally, you would not see their availability in your calendar.
However, the doctor has also activated a special feature within their account to show their availability to practices not on their personal list of employers (shows them to practices outside their radius) – but to not show those practices their contact details
Which means you can see the doctors availability and request the doctor via the system but not contact them directly