App to receive text messages from another phone number?

No app can forward another phone's texts to you from a number alone. SMS arrive at the SIM, and only the device holding that SIM — or software installed on it with permission — can read and forward them. Anything advertising a number-only setup is a scam that will ask for a payment card.

The legitimate ways to get a copy of texts:

1. Call forwarding plus a second number — carriers can forward calls to another line; SMS forwarding is possible on some plans (e.g. Google Voice can mirror texts for a number it owns, not for someone else's).
2. A monitoring app on the Android device — reads the SMS store and syncs to a dashboard. Requires physical installation and the user's consent.
3. A shared Apple ID — iMessages and SMS relay appear on every device signed into the same account (Settings → Messages → Text Message Forwarding). This is Apple's own sync, and it is why families with young children often share an ID.
4. Dual-SIM or an eSIM transfer — moving the line itself, which requires the carrier account holder.

For a child on Android, option 2 is the standard: the app is installed, the child knows, and the dashboard shows threads. For iPhone, option 3 or nothing.

Where our app fits. Ultimate Phone Spy runs on Android 4.0+, Windows and macOS (supported devices) and shows SMS/MMS threads for an installed, consented Android device. We do not offer number-only forwarding, because it is not technically possible.