Under certain circumstances, a mobile phone could be programmed to go into silent mode. For instance, if a number of calls has gone unanswered recently, either the owner of the phone is not present or does not wish to be disturbed. The phone can then be silent for a while, since continuing to ring out loud is clearly doing no good.
Mokalus of Borg
PS - With suitable software, this would be pretty easy.
PPS - As usual, I have not done my research, so I don't know if some phones do this.