Systems like GateGuard® (which is only $49.99/mo) will ring your phone, iPhone, Android, or tablets which you can put anywhere in your apartment.

It's a massive waste of money to install on-wall units. (Those who want on-wall units can get them from GateGuard, and they can use existing wiring saving you many thousands on labor. Or they can mount a Samsung tablet to the wall and get the same functionality from the GateGuard app).

Hundreds of buildings across NYC and North America use GateGuard. It's impact resistant, works in all weather, is insured (if it fails they'll replace it free!), and they install it themselves so you're not paying thousands in markups to a 3rd party installer. You'll see them all over NYC, the glowing blue screens.

Https://teman.com/GateGuard

(if the history of this board is any judge, sadly you'll now see ugly replies and false accusations from guys trying to push cheap Chinese knockoffs and sell their expensive installation services... Buyer beware. Speak directly to the sources!)