Ok, I want to know when person X (any person using a mobile phone) is reachable, so I can voice call him.
Some fascinating observation, just because you have GSM service, that does not mean you have data service (especially when one is at the edge of network coverage, or e.g. while roaming).
With SMS, you send a nice message with "delivery report" enabled, and the next time that teenager with behaviour problems is reachable, your phone will notice you via the delivery report. Next step, call said teenager (that sadly happens to be related to this parental unit), and discuss your concerns.
Basically, SMS is standarized part of the GSM standard for decades now. With rather exact semantics. Apple tried to implement a short circuit this with their own service, and by doing so broke the semantics of this (in some way nowadays very simplistic looking) service.
So it's absolutely correct that they get sued. The sad reality is that repeated inability to receive and/or send messages can cause significant damage.
(send: "Hi Boss! My kid is in the hospital, will contact you in a couple of hours" => depending upon the boss you might find yourself out of work if you disappear for a couple of hours, and hospitals tend to have poor reception. Same thing the other way: "Hi John! We do have an issue here with VIP customer, please call me ASAP".)
I know, that might sound over the top, but I have lost a contract in a similar scenario (you only need to have a team lead with a bad temper, that has a bad day).