Most families are dispersed over a large area during the day. Parents are at work and kids are at school. Having everyones telephone number in one place will allow you to more easily contact each other during a disaster.
Phones tend to be jammed during a major incident. It is often possible to make long-distance calls out of an affected area, so even having prepaid calling cards in everyones wallets is a good idea with a common out-of-state phone number printed on it.
Finding alternative means of communications are recommended. Ham Radio, CB, GMRS, and FRS are all alternatives to the telephone system. Create a family band plan of 3-4 frequencies/channels to attempt to communicate on.
Don't forget Am/FM radios too. Your city should have a Primary and secondary Emergency Alert Stations.
The internet is also a good communications medium. By setting up a family blog now on a free service like Blogger and giving everyone the URL, they can leave comments there, if everyone is separated, to let everyone know they are okay. During Hurricane Katrina many families were separated and this is an easy way to contact each other to come together again.
Others may need to know your status and having their numbers are important too; like work and church leaders.
Make a list in a Word Processor and print out copies for everyone.
Make a list of everyones phone numbers, and addresses that can be useful. Include everything:
The add things like:
Apartment owner/HOA president
Real Estate Agent