It is possible to select for each participant which assignments are possible.
I would welcome an additional field for the possible frequency of assignments.
e.g. some participants can only participate 2 or 3 times a year.
At the moment I note this instead of the phone number, but a dedicated frequency-per-year field can be used in the assignment process to filter for this setting.
Thanks for the input
It is not possible at the moment. It has been noted and will be passed on to the development team for considerations in the future.
The wish list is long - but we get some things done in each version 
/Ringer