The behavior I am seeing is that only the primary contact method (and NOT additional contact methods) is processed when multiple users are on call at the same time, on the same calendar.
User1 has contact methods Email1 + SMS1
User2 has contact methods Email2 + SMS2
Case 1 -- If both users are on call on the same calendar and I notify a user from the calendar: Email1 + Email2 are sent (no SMS's)
Case 2 -- If only User1 is on call, then Email1 + SMS1 are sent