Why do my text messages go through when I can't make phone calls?

[AC Question]

I have no cell phone signal (calls cannot be made or received) but text messages are sent/received. I haven't installed any apps for this purpose, so how is this possible?

 

[ManiacJoe]

Most (all?) phone come with a basic texting/SMS app.

SMS messages are transported using the otherwise unused parts of the "house keeping" data packets that are always flowing through the mobile system. Voice calls require a higher signal strength than the "house keeping" and SMS packets.