I am in need of some software, database or coding to check phone numbers which are entered into a form. These check is to verify that number exists etc.

I don't think you can verify if a number exists. You can, howver, make sure that was is entered is a valid number (and not just random characters). By valid, I leam formatted correctly. This can be done using Regular Expressions.

here is a regex for matching us phone numbers...

^(?:(?<1>[(])?(?<AreaCode>[2-9]\d{2})(?(1)[)])(?(1)(?<2>[ ])
|(?:(?<3>[-])|(?<4>[ ])))?)?(?<Prefix>[1-9]\d{2})(?(AreaCode
)(?:(?(1)(?(2)[- ]|[-]?))|(?(3)[-])|(?(4)[- ]))|[- ]?)(?<Suf
fix>\d{4})$

Matches:
333-4444 | 222 333 4444 | (222) 333-4444

Non-Matches:
222333 4444 | 222-333 4444 | (222)-333 4444

http://regexlib.com/REDetails.aspx?regexp_id=536

I agree with Foobar that you can't check to see if the actaully is hooked up. You can only check to see if it is valid.

I would suggest client side checking rather than server side as it saves server time.
I'd use some JavaScript functions. With 3 or possibly 4 html textboxes (1 for areacode, 1 for the prefix and 1 for the suffix, maybe one for extention) You'd just need to check if they're numeric.

I'll post some code later.

Personaly I would stay away from client side scripting as there are a few users who turn off JavaScript. Something like the regex above would not put that much of a strain on the server to be of any significant difference.

Here is how I would do it with php...

PHP Code:

$strMatch = ereg('^(?:(?<1>[(])?(?<AreaCode>[2-9]\d{2})(?(1)[)])(?(1)(?<2>[])|(?:(?<3>[-])|(?<4>[])))?)?(?<Prefix>[1-9]\d{2})(?(AreaCode)(?:(?(1)(?(2)[-]|[-]?))|(?(3)[-])|(?(4)[-]))|[-]?)(?<Suffix>\d{4})$', $_POST['phone_number']);

if($strMatch) {
    // It matched, do something here
}else{
    // It did not match
}