The function of Only You is to allow whitelisted callers to get through to you, while everyone else is diverted to voicemail.
This can be especially useful if you use the same phone number for work and personal purposes.
A sensible feature is the provision of a timer so that normal operation is restored shortly after the end of a meeting or movie, or even when you get up in the morning.
The basic app is free, but only allows two calling numbers to be whitelisted.
In-app purchases allow this to be increased to five ($US0.99) or an unlimited number ($US1.99).
Only You is available via Google Play.