The answer to this question largely depends on which medium you are advertising in.  In print ads, the vanity number should be large and bolded with its numerical equivalent accompanying it in very small type.  With television, radio and billboard ads, you should ONLY use the vanity number since you only have a few seconds to make a lasting impression on potential customers.  With online advertising, vanity numbers shouldn't be used at all.

You might argue that not using the digits at all will frustrate PDA users who don't have actual digits on their BlackBerries or Treos.   Perhaps people might get frustrated figuring out how to dial a vanity number, but they WILL remember your number.  They'll actually probably even remember your number better than they would have otherwise.  Why?

Think about it.  If you're looking into getting hardwood floors for your home and hear a great 1-800-HARDWOOD ad on the radio, will you not call simply because you have a PDA?   Of course not.  You'll figure it out.  

In fact, you will probably mention your frustration to several of your friends and explain how you figured it out.    They in turn might discuss your story with their friends and create more business for your company. Perhaps they'll even call 1-800-HARDWOOD as well or go online to figure out how to dial a vanity number on a Blackberry. Wow, what wonderful viral marketing!

By the way you can dial a vanity number on a BlackBerry simply by holding down the "ALT" key while using the keypad to type the name in the vanity number you're trying to dial.  The BlackBerry automatically sends the right tone to match what you've typed.    Pretty easy, eh?  

So as it turns out, vanity numbers can increase your sales in ways you perhaps never even imagined!