@MediaActivist @MediaActivist just don't want you stewing over it on your own.
It seems unreasonable on the face of it for a few reasons.
I've added an update to the wayback machine for their page admitting that scams by phone contact exist so there's a contemporary proof at the time of their change towards you.
Other things that seem important at the moment:
1) reasonableness of expecting you to believe that a HMRC telephone contact to be genuine.
2) reasonableness of changing terms after only ONE attempt at contact.
3) the important FACT that you receive a LETTER from the HMRC about missing a phone call says something about a) how to contact you, b) how to provide and receive information in a recordable way, for both parties c) that it is a more reasonable context method to assure it is a genuine communication.
4) the reasonableness of using one form of communication because it is probably cheaper to call than send a letter for one party.
Don't forget to save correspondence from them.
Hope these are helpful at a stressful time. They are trying to pressure you, but what they want and what the law allows are not the same thing and things like standards of "reasonable" can count.