implement voicemail code sharing challenge

[kra]

Hash 4 digit number -> 4 digit number

User selects challenge
A random existing voicemail box with a greeting is chosen, not the user's (ie has passed that challenge)
A message is left in that voicemail box, "attention the code for [user greeting] is [hash of user's VMB]"
User hears "Your code has been told to [recipient's greeting]. Tell them to check their voicemail!"
User hears "enter code"
correct code -> success

[kra]

We don't want a new random recipient each time, because then users would be receiving several and duplicate messages. But we can't hash the VMB for the recipient because it's sparse and grows. Could just do VMB modulo population, then it only changes when a new VMB is added?

But we still would need to keep track of who had been a recipient for which senders, otherwise recipients can get multiple messages from the same sender, this needs a new storage, which I don't want to do.