I vaguely remember my parent's friends having this by smuggling it from overseas.
How it works was that you attach a special antenna to your landline and place it somewhere on the roof.
In your car, there's also a hub/receiver where it can create a connection to this local network.
Lastly, the mobile phone you carry around makes a connection to your car's hub.
Anyone remember this? I can't seem to find any information on this.