Have an IVR system
From David Hamner's comment on a Calls issue:
"Auto answer the phone and ask for input:
- Press one (or enter pre-setup pin) to ring phone
- Press two to leave a message
- PIN Code to start phone alarm and or IM GPS (lost phones)
- Custom greeting (Custom menu options/audio)
Give known callers a PIN that lets them ring straight through to your phone. They can then input your number into their contacts as [number][pause pause pause][PIN] to make it seamless."