Today, a little bulldog ran into my garden. It had a collar on, and didn't look distressed, but it also looked lost. I've never seen the dog before, and no one seemed to know where it came from. No one came running after it either. The dog was running full speed around and checking everything out. I tried to lure it indoors.
The dog had a collar on - and I could see that the collar had a tag on it too. Unfortunately, the side which would have had the phone number on it was facing away from me - so I couldn't see a number to call. The dog was also friendly enough but skittish - every time I made an attempt to look at his tag, he would leap away. By the time I went in to get some food to tempt it with, the dog had taken off and couldn't be found.
This then is the problem with tags. If your dog or cat is shy, chances of someone being able to read the tag is really a coin toss. If the side with the number on it is facing outwards (AND the animal stays still long enough), then your contact information can be read. However, there's also a good chance that it ISN'T facing the right way - and in that case, the tag does no good at all.
One solution around it is to put your contact details on the collar itself. Here's an example of one of the collars sold. The other way of doing this is just to get a permanent marker and write your number in large letters on whatever collar you're using. As long as it's a plain collar, this works just as well. Either way, people can easily see your contact details and call if they see your cat or dog lost.