Technically speaking, chewing gum may not break the Communion fast since the gum is not food, but it is inappropriate decorum for church and can cause fellow congregants to be scandalized. If you would not chew gum in a court of law out of respect for the dignity of the proceedings, you should not chew gum at Mass, which is the re-presentation in time and space of Christ’s sacrifice on Calvary.
As for cell phones, ordinarily speaking, they should be turned off at church. If a congregant is someone who is on call for emergencies, perhaps he can wear a vibrating beeper and sit near an exit. If he’s paged, he can leave without disturbing anyone and use his cell phone to answer the page. If someone who is not on-call but may be needed by others feels the need to check in, perhaps he can call in immediately before and after Mass outside the church.
