[M95D]

It's actually moderately easy: get 3 phone numbers. One you make public (#1), the one that everybody knows. The third one (#3) you never make public and it's the one you take with you in your mobile phone. It's preferable to get this number illicit - not in your name.

Put #1 and #2 SIM cards in two WWAN modems in a device (laptop?) you always keep at home, or at work if you don't want your home location known. Cross-connect the two modems by software (Asterisk?) such that if a call comes in from #1, it's forwarded through #2 modem to #3 (your mobile phone) and if you dial #2 from your mobile phone, you get a dial tone on #1 modem.

Disadvantages:

  - No call ID. You'll never know who calls you and can't immediately save their number without looking at the call logs at home.
  - You must store all phone numbers with #2 as prefix, a pause, then the actual phone number as extension.

PS: It's possibly even simpler than that. Forget #2. Use a VoIP app to connect to laptop's PBX from the mobile phone. That way you can even block all calls in/out from your mobile phone (PIN2 required) and only allow internet.