How to read someone's text messages without their phone?

You cannot read anyone's texts "without their phone" in the sense that phrase implies: there is no remote read, no number-only lookup, no page where you type a phone number and see a conversation. Real access needs either credentials to the person's account or physical installation on hardware you administer.

Account-level, which is not a hack. Google Messages for web mirrors an Android phone's SMS threads in a browser, but only after a QR scan on that phone and while it stays online, with a permanent "Messages for web is on" notice on the handset. iMessage and SMS on an iPhone sync to Apple hardware signed into the same Apple Account — a Mac or iPad — which is why "read my kid's texts from my laptop" really means "put the Apple Account on a device I own", and needs the child's cooperation or a supervised child account. On iPhone, Screen Time and Family Sharing tell you whether contacts are allowed at all, not what was said.

Device-level, on hardware you control. This is our method, and the only one that yields message content without you unlocking the phone afterwards: install once on the Android handset, with the user informed, then read SMS, MMS and call logs in the dashboard (SMS tracker). Ultimate Phone Spy runs on Android 4.0+, Windows and Mac; $69.90/month, $40/month quarterly or $16/month annual (pricing). No iPhone support, no remote install, no iMessage — anyone promising all three is describing a capability iOS does not grant.

Carrier records are metadata only, and content requires a subpoena. Installing this on another adult's phone without consent is a crime under the Wiretap Act and ECPA, the Computer Misuse Act 1990 and stalking statutes, and the output is inadmissible. It is lawful on a minor's phone you administer, or a company device under a disclosed written policy. Fuller breakdown in this thread.