I think cancelling anything in life usually is preceded simply by " I'm sorry I can't make/have to cancel" and that goes a long way whether direct or through a receptionist/secretary etc and unless a family member or close friend I think that is usually adequate-rescheduling at the time or updates /estimates of time to rescheduling may follow
Therapists are extremely personal to us and it is easy to expect more but it is their job and they have many clients/hospital/teaching commitments-blatant disregard /disrespect in any interaction is unacceptable and therapists are not exempt from that but neither should they be held to higher standards than say my oncologist who regularly wasn't there and I found out only from the whiteboard in the waiting area accompanied by approx waiting time 4-5 hrs-life happens-she had my trust/my life in her hands