The best emergency and occassion use phone I've found is Page Plus prepaid.
www.pagepluscellular.com
They are a verizon wireless reseller and use their network, which is going to be the most extensive coverage across they country. They offer rates from 10-15 cents per minute. Their home coverage area is the Verizon native coverage which includes the most populated areas of the country. They roam for about $1 per minute outside of that. But at least you will have coverage.
Since this is an emergency phone you want the least possible per month charge. You can buy a $10 card that lasts 120 days. There is a $0.50 per month charge. This will cost $2.50 per month or $30 for the year. They will activate any Verizon phone. You can get an inexpensive Verizon phone off ebay. Then buy a $5-10 activation kit from ebay.