In the context of preferring an easy scheme to analyze assignments per user. To respond to question like:
How many calls were assigned yesterday?
Which users were assigned to X call?
Month, week, day, hour, with most than X assignments.
Total time in assignment for X call.
The question is what are the most common application user queries and this should define the schema.
If your application need to present call details with participate details it make sense to store it in an array if the amount of participants is not large.
If I would go with the array approach I would do 2 mandatory things:
Store the amount of participants in the root level and keep it updated as the array grows as well as the call time/duration in the main object.
2.I would index users.user to use miltikey searches for user calls.
If there could be houndreds of participants per call I would not keep it in an array ,you can explore a hybrid solution using the Outlier pattern