Cell Towers can handle a limited number of simultaneous connections. A cell call requires a dedicated connection, once all the local towers you can reach fill up, you wont get cell service until someone releases theirs and you get one back. Advances in cell communication schemes have drastically increases the number of connections towers can support, but they're still limited.
SMS (text) messaging doesn't require a dedicated line and often use seperate channels from the primary cell service, so "service" is more open with the system sending and recieving texts whenever bandwidth is available (it can take a lot longer than normal for the message to go through, but it goes through) If your phone allows "group" messaging - set up a group for your emergency contacts and send out text messages to indicate rally points and confirm safety.