You can use Google's Firebase Cloud Messaging API to send out (non-sms) messages free tier. You'd have to create an app and register for messages on the mobile, but then you can present the message in any format you want (simple text, decorated, custom, etc.). But, the sender side (host server) is really simple (http POST with data - curl suffices for example). I've only done it for android. Probably, more work than you want. There's a learning curve, but it is pretty much free to use.
The whole TCP back to server on a mobile platform & wait for data gets more complex because of sleeping, memory consumption, keep alive, also queuing for when the phone is off-line, etc.. You are basically duplicating all the existing effort that the mobile platforms already include.