Short version: getting full text transcripts from the carrier is usually a dead end.
In most countries/carriers, you (even as a legal guardian) can typically only get:
- Billing records and numbers contacted
- Maybe time/date of messages
- Not message content unless there’s a court order/subpoena
So in practice, yes, you almost always need something on the device itself:
- Built‑in parental controls (Apple Family Sharing, Google Family Link)
- A monitoring app (e.g., Spynger) that logs SMS, some messengers, and web activity
I’d start with the free native tools, add an app only if that’s not enough, and be upfront with your teen about the monitoring to avoid a big trust blow‑up later.