You can get someone's text messages in exactly two lawful ways: the messages arrive in an account you control, or the phone belongs to you and you administer it. There is no app or lookup site that pulls SMS off a phone you have no access to — that promise is either a fake dashboard or an illegal install that will be found on your own devices.
Free first-party routes cover most real cases. On Android, Google Messages for web mirrors the phone's SMS and RCS threads in a browser, but only after a QR scan on that phone, and the handset then shows a permanent "Messages for web is on" notice — consent and proximity are built in. On iPhone there is no web client and no export of SMS: Messages in iCloud syncs only to Apple hardware signed into the same Apple Account, so that is account access, not a hack. On a family plan, the carrier portal shows message and call metadata — number, date, duration — never content; message bodies need a subpoena or the account holder's written consent.
Where a monitoring app fits. On a minor child's Android handset you administer, device-level monitoring is legitimate, and that is what our software does: SMS, MMS and call logs in one dashboard (SMS tracker, system requirements). Ultimate Phone Spy runs on Android 4.0+, Windows and Mac at $69.90/month, $40/month quarterly or $16/month annually (pricing). It does not support iPhone, does not install remotely and cannot read iMessage — iOS grants no device-level access, which is why any competitor claiming iPhone text reading without the device is lying to you.
The line. Reading an adult partner's or ex's messages without consent is a crime, not a grey area: Wiretap Act and ECPA in the US, Computer Misuse Act 1990 in the UK, GDPR and stalking statutes across the EU. The evidence is inadmissible in court anyway, so it fails on its own terms. The lawful paths are written consent, a shared account they know about, a licensed investigator, or a court order in a divorce or custody matter.