Your phone may not be ringing due to a number of similar reasons. Ensure that Do Not Disturb or Silent Mode is not turned on, these will not allow it to give notifications. In addition, make sure that the volume is turned up and that it has the ring tone set. Ringing may sometimes be compromised by a software glitch or old operating system. Reformatting usually clears up short-term problems. In case all of these fail, then it may be a hardware issue such as a defective speaker. Know more about phone not ringing