If you ever learned anything about the TCP/IP protocol, you will have heard of 'connectionless' vs 'connection oriented' protocols. A connectionless protocol is like a note tied to a balloon and released: it's just thrown out there and you assume it will reach someone, but there's no subsystem between sender and receiver to make sure contact is established. A land-line call is representative of connection-oriented communications: both parties can confirm that the other guy is there, and if one party disappears, the loss becomes evident to the other.

A regular cell call is connection-oriented, and while you're on that call, the cellular system 'real estate' is dedicated to you and only you. Since the resources assigned temporarily to you can't be shared by other users while your call is happening, the whole system can saturate.

A text message, and to a lesser extent a PTT conversation, is sent by means of a best-effort or connectionless method. Your text message is released into the system and may or may not come out the other end - the system doesn't do extensive checks to make sure both ends are engaged. Your text message may also wait in line while system resources are freed up. This can't be done with standard voice calls, unless the cellular carrier wants to sacrifice quality of service.

Furthermore, like any computer, the cellular system requires much less horsepower to send the seven characters "I AM SAFE" than it does to send a recording of a human voice saying those words. That's why the text of Moby Dick can fit on a floppy disk, but the book-on-tape would fill several CDs.