IAS Not sending out appointment letters?Prevention

have heard that IAS do Not send out assessment letters sometimes for claimants ( for example NOT sending a letter in the postthat the claimant has a face to face on xyz date)

Which means that the claimant can miss their appointment, on the basis the were still waiting for a letter, which didn't even come?

1) Is this true? Do they do it often?

2) In the worse case scenario, if this does happen, and the claimant does not send an appointment letter,

then how can this missed appointment be Known of, so that it is actually NOT missed, and attended by the claimant?

in other words how can missing this appointment be avoided?

3) What is the usual waiting times for IAS, from when received your claim, to actually give you a letter to say you have an appointment?

How many weeks?