Setting value from higher in property chain

I'm trying to update a specific value in an object (in Ember 3.12) using the following command:

set(this, 'activeBooking.directDetail.phones.firstObject.number', guestPhone);

This works, but since number isn't the key on a computed, it doesn't update the rendered value on the page, so I'd like to target phones instead, which is the key on a computed. How could I set the number value by hitting phones? Thanks in advance.