Some countries require ID/passport to just get a phone number. This could potentially be sidestepped by getting a Twilio number or Google Voice number, but that causes it to be far from frictionless.
Unless if ISP positions itself as rigid security service devoted to provide you security features and some famous people recommend using it in order to transfer sensitive documents.
Public internet access, both in the form of hotspots and computers, are perfectly usable (if not terribly secure). But you cannot use Signal with a payphone.