To expand on this a bit - if you live in an area with EVDO digital wireless service - EVDO runs through a separate radio & delivery system from the CDMA voice system. CDMA is a circuit switched system. That is, each call requires a separate fixed circuit. Generally speaking, any network has a certain fixed number of circuits that have been installed to handle a peak number of calls (called busy hour call attempts). If an emergency drives the number of calls above the available number of circuits, the network jams.

EVDO is a separate RF system that feeds a separate IP network which avoids the circuit switching part of the network. There are usually fewer users, and the backhaul network is bigger. When you send a message on an IP system, it can sit in a queue to be transmitted when the network becomes available, even if only for a few seconds.

The practical upshot is that your odds of getting a msg out on an EVDO network are probably better than making a voice call in a jammed system.