Is this a better way to validate a text box for mobile number only?
Again, properly indent the code. The code was unreadable to the point that we broke our own rules and edited the code in question just to be able to review it.

Other than that, the biggest issue with this is all of the magic numbers. They mean absolutely nothing to Mr. Maintainer. Normally at this point, I tell people to create an Enum, but in this case, there's one built in. Use the Keys Enumeration to replace all of those hardcoded numbers.