Spy on a phone without installing apps

I was having dinner with some friends yesterday, and one of them mentioned it's possible to spy on someone's phone if you just know their phone number. He said just with the number you can get access to Whatsapps, phone activity, etc.

I was researching it online but only found it was possible if you first got access to the phone and installed an app. Then the tracking was possible, but not without prior installation.

So is it actually possible …

apps phone privacy spy