Hello Sheryl-Lynn,
Sumit from the Firebase team here! One option that might work depending on your user base that won't require additional work is using a combination of Analytics audiences and using audience targeting with Firebase Cloud Messaging.
So depending on how your users are distributed, you could create audiences by say age and gender in Analytics, and then create scheduled FCM messages targeted to each audience. It might not be an even distribution across all your users for each age / gender, but if the numbers work out for the call volume threshold it can be one potential solution.
Hope that helps!