Nick Herbert from the U.K. was having a hard time with his son not responding to his calling or texting because the phone would always be on silent.  So, he came up with the app ReplyASAP, which overrides the silent setting on the phone and emits a loud noise until it's acknowledged.  Both parties have to have the app for it to work but Nick doesn't use it unless it's important.  The cost depends on how many people you need to contact but can come in handy i other scenarios, as well.