How can I distinguish important calls?

I'm a freelancer and have a somewhat erratic work schedule, so it is rather difficult for me to tell people at which times I'm available, which means that occasionally people call me while I'm at a client's site.

I'd rather not blanket reject or ignore calls as occasionally there is something urgent that needs my attention.

Now I'd like to have my phone on silent while I'm at the customer site, but ring when the same person calls another time within five minutes of being ignored.

Is there an app that can do this already, or can it be done as an app within the normal permission framework?

Answer

Harass Me by Samuel Tardieu available from Google Play Store does what you want:

Tired of missing extra-urgent calls because you forgot to switch your phone off silent mode?

This application will turn the volume on if the same number tries to call you 3 times in 3 minutes (configurable).