Think it goes like this
Already a fisa on Kislyak cuz he's russian.
Kick out Russian diplomats.
Assume Kislyak will call Flynn to say wtf
Tell FBI to get the call since they should already have it due to Fisa on Russian.
Can't be that many phone calls to sift through since Flynn in DR
No need to unmask hence no unmasking around the time of the call
See Bongino episode from last Monday
So who would Kislyak call first when Russian Ambassadors get the boot? State department?