What should I do when one (or more) of my planned references are on leave (or otherwise unavailable for some period of time)?
I think there are 3 (or possibly more) aspects to this question:
They are reachable using any method of communication
Depending where they are and what they are doing, they may not have as consistent access to these forms for communication, or ability to talk, as one might expect from a desk-job. If they say it's okay to contact them, it may just be that they're trying to be helpful and/or are underestimating how reachable they are.
They are unreachable using some / all methods of communication mentioned by the interviewing company, but are reachable using some others (e.g. the interviewing company requested the phone number and email address of the references, but they are only available by phone, or perhaps only using Skype)
Here is mainly comes down to whether it's appropriate to provide contact details different from the ones the interviewing company specifically requested, and how this should be approached.
They are completely unreachable
Here is mainly comes down to whether it's appropriate to request that the contacting of the reference be delayed until they come back from leave, and how this should be approached. This also applies to the above two cases.
The leave may or may not end after the point at which the interviewing company planned to contact them by, which, I, of course, don't know.
Which of these should affect my choice to use them as reference as opposed to a fallback reference?
Should I mention the fact that they are on leave (and perhaps include their return date, assuming I have this information) in any of the above cases? If so, should I provide the details of a fallback reference in case they wish to contact the reference prior to coming back from leave and/or using a form of communication which they don't have available?
Should I instead try to find out when they plan to contact them to see if it's during or after the leave?
PS - This question isn't about finding out if they're on leave and how / whether I can contact them - it's assumed that I already have this information.