10:00 am ET might be unsuitable for MST time zone
When planning a call between Eastern Time and Mountain Standard Time, you need to consider time difference between these time zones. ET is 3 hours ahead of MST.
It is 10:00 am in ET. This does not fall within the span of usual working time between 9:00 am and 3:00 pm in MST, but it still might be suitable to arrange a meeting, since it would be 7:00 am in MST, which is between 7:00 am and 11:00 pm MST time.