Clearly a mobile phone can make phone calls and send text messages as a key function. Apps can access these functions, but there are restrictions.
A phone number can be included in the text of a content page. The user does not have to type the number in, just tapping on the number will bring up a dialog box asking if they want to dial the number (or cancel if they tapped by mistake)
Cost and technical implications: Including phone numbers in content is standard, no additional costs (apart from the cost to the user of making the call)
Sending text (SMS) messages may seem like an easy thing to do, as all phones can do it, but there are implications to consider. If apps were allowed to send SMS messages, then any app would be able to send spam messages from your phone. You might install a game app, for example, and ten minutes later all your friends would receive a text message from you saying what a great app it is.
For this reason alone, Apple and Google do not allow apps to generate SMS messages. It is possible to create an app in which the user actively sends an SMS message, but it has to be initiated by the user directly.
This limits what can be done slightly. It would be impractical to make an app send a text message on its own without user interaction.
You can arrange to send automated text messages by setting up an intermediate server with which the app communicates. All messages are then send from the server, and can be subject to regulatory restraints. It is clear that the messages came from the server’s owner, and the server’s owner pays the charges accordingly.
Cost and technical implications: Requires customised development. Medium cost to develop the interface to send a single message.
To send automated text messages is technically possible to set up, but significant cost implications for additional infrastructure, and there are almost always better ways to send notifications to people.