Ever since I discovered custom cell formatting, I have enjoyed using it, especially for phone numbers. However, there was one problem I ran into.

As long as I kept the phone number formatting to a base phone number, which has a predictable number of numbers. However, what about extensions? That's where I'm running into issues. Apparently Excel cell formats can contain conditionals, which is awesome. This conditional almost does everything I need it to:

But plenty of extensions are more than 2 numbers. Some are three, others are four. I've seen extensions as many as 6 digits long. However, when I try to modify the conditional formatting I've seen, I get errors:

I'm clearly doing something wrong, but I'm not sure what. I read that past versions of Excel (2003) weren't able to handle more than 3 conditions (4 if you include the default), but that later versions (2007, 2010, 2013, etc...) could handle more than 3. If that's the case, then it's almost certainly my mistake. But, since I followed the rules I noticed in the original, I can't, for the life of me, seem to figure out what went wrong.