It's possible to do many things in MIT's app inventor, yet, there is no clear way on how to receive a phone-call/SMS or even showing the calling number ... is that possible?
You can't receive a phone call with App Inventor, there is the PhoneCall component, but you can only Use this component to dial the phone and make a call.
There is the Texting component, which can send and receive SMS and also display the sending phone number: When a message arrives, the MessageReceived event is raised and provides the sending number and message.
Please see the reference documentation for more info.