It would not be a phone nor an app - instead, it would be a very distinctive object that would be difficult and expensive to mock up and it would function as a one-way alert: if triggered, an actual lawyer races to your physical location. The trigger would be un-cancelable.

Instead of tying it to your own, specific lawyer, there would be broad agreements that any lawyer could enter into placing them "on-call" for a legal-bat-signal and then the closest (and fastest) one gets the call.

This is quite dystopic, however: basically, very rich people would have a (figurative) hand-grenade that they could pull out and dare LEOs to make them press it.

Further: persons with resources represent one of the best sources of pressure to change these systemic problems. If persons with resources exempt themselves from this kind of overreach then we lose that pressure.