It's not tethered to your phone. It does work with the iOS version of Signal. It's really easy to set up, too. Everything in your comment is incorrect or out of date.
Compare that to Wire, which has much more convenient multi-device support, but accomplishes that at the expense of keeping the entire active graph of communicating Wire users in plaintext in their servers on AWS.
Both Wire and WhatsApp (which have comparable security models with some marginal tradeoffs --- WhatsApp metadata is in Facebook's custody, which is not great, but Wire doesn't have Facebook's extremely competent and well-resourced security team) are reasonable choices for secure messengers.
Telegram is not.