Hey LMan! Please use the Question Tags to state which version of Twine and which Story Format you're using, as the answers will differ. Also, it's much easier to help you if you paste in the existing code.
In Sugarcube, you could use the <<if>> macro.
<<if not $callMike == "y">>\
<<if not $callSarah == "y">>\
(The \ back slashes at the end of the code lines prevent blank lines appearing in the game.)
If your variable is just a yes/no, it might be better to use a boolean. Here's how it would work. When the player calls Mike, you would:
<<set $calledMike to true>>
Then, in future passages:
The player has called Mike.
<<elseif not $calledMike>>
The player has not called Mike.