Probably the difference is that you have not used server side validation before in those other forms. Server Validation uses session variables to retain the previously entered values so that the user can start over from the form they had filled out when a particular field fails validation.
Really once you get client validation working properly it will probably appear to work as you want since the client validation will catch all of th errors sever validation won't have to and the reset button will work as you expect. This is a result of the server validation running because you client validation is throwing an error.
Simply adding a form element to a working page will not break it. When you are making changes, test your changes as you make them. If things break, ctrl+z to undo and keep a working copy of the page. Having a working backup will help you analyze what you actually did to cause problems in the first place and then you will know exactly what you did to cause the error.
In this case the current error was actually caused when you removed the CAPTCHA. You failed to remove the code in the onSubmit event of the form validating that field:
WAValidateRQ(document.emailContact.Security_code,'- Please enter the text as it appears in the image above',document.emailContact.Security_code,0,false,'text');
That code is now causing errors and the client validation is not able to run at all as a result. This has nothing to do with the phone number field which is why you were pulling your hair out trying to find something wrong with the phone number field.