There are times we all have experienced when the voice cell phone network is so busy you can’t get your phone call through to someone. This sometimes happens occasionally around 8:00 am or 5:00 pm in the day because so many people are making cell phone calls. This can and does happen when a major event happens, like 911, or a sudden storm coming. It can and will happen if we had an earthquake or a major terrorist attack.
What can you do? This is the time to text message even if you are not a texter. Voice and texting are sent on different frequencies. There is a much, much better of a chance that your text will get through when the voice cell lines are not working because of large numbers of users.
This of course will not necessarily work if phone towers are out of commission. But it may just work when you need it.
See this SciAm post for a more technical explanation of how all of this works.