If many EM signals exist in same time/place, when would interference occur?
If they were all on the same frequency, at the same time, and using the same modulation scheme, then yes, there would be interference.  But cellphones (and other services) use methods like multiple frequency channels, timeslots and "Code Division Multiplexing", also known as "Spread Spectrum".  These techniques allow many transmitters to share the same frequency band.  

In general terms, phase has little to do with it.