The Prophet Mohammad (pbuh) taught us that once the Khatib (Imam giving the Friday sermon) begins his Khutba (sermon), one is not allowed to distract himself or herself in any action or talk. It is such an important rule that even asking someone to be silent during a Khutba puts one at the risk of actually losing the reward of the Jum'aa (Friday) prayer. Of course, this does not stop some Muslims from engaging in greeting everyone they pass by, as they enter the Mosque late while the Khutba is going on.
Every once in while, while I am giving a Khutba, someone's cell phone starts ringing. Most people rush to silence it. I can see the guilt and embarrassment on their faces. However, I also witness the rare cases of those who reach to the phone and answer it. I vividly remember the time a middle-aged man sitting in the 2nd or 3rd row answered his cell phone and angrily and discretely (at least he thought he was discrete) whispered to the caller (who I guess was not Muslim): "I hope it is something important. I am in the middle of a sermon and the angry preacher is looking at me now!"
I have to admit, while this was a "what's up with that" moment, I struggled hard to keep myself from laughing in the middle of the Khutba.