how to listen in on phone calls?

Recording or listening to a call you are not part of is illegal nearly everywhere, and the technical picture matches the legal one: modern phones do not expose call audio to third-party apps.

The legal frame first, because it decides everything. In the US, one-party-consent states let you record a call you are on; all-party-consent states (California, Florida, Illinois, Massachusetts, Pennsylvania and others) require everyone's agreement. Recording a call between two other people is a federal wiretap violation regardless of state. In the UK, recording your own call is generally fine for personal use, recording someone else's is not. Across the EU, consent rules vary but covert interception of a third party's call is criminal everywhere.

What is technically possible:

• Recording your own calls — apps do this on Android, and it is lawful where one-party consent applies. iOS blocks call recording through third-party apps entirely; the iPhone's built-in recording (iOS 18+) announces itself to both parties, which is Apple's compliance choice.
• Call logs, not audio — which numbers were dialed, when, and for how long. That is what monitoring tools actually capture on a device you administer.
• Ambient recording — some Android monitoring products will record the microphone around the device. This is the capability that gets products banned, and in most jurisdictions it is unlawful to use on anyone who has not consented.

What is not possible: listening to a live call on someone else's phone, from a number, or through a website. Every product advertising that is fraud.

If the question behind the question is "what is my child doing on the phone", call logs plus message monitoring on a device you own answers it lawfully. Ultimate Phone Spy does call tracking and recording on Android, Windows and Mac under a visible install (call recording, call tracking), and it is explicit that it is for legal monitoring of devices you administer, not for intercepting other adults.