is it possible to make an iphone app that will only call to one number no matter what number was dialed?
you could write an app that has some number you have hard coded into it. When the user initiates some function you have programmed, your application will hand off the request to actual phone application with the number you specify.
If you are asking whether you can override the built in nature of the phone to circumvent the original dialer such that only one number is able to be called, I dont believe this is possible. YOu have no control over native applications.
If you're asking about whether you could override the telephone function so that it would dial call your number, no matter what the user dialed, that would be a horrible security violation. You could prevent emergency calls or make the phone dial some high-cost 1-900 number. Of course Apple won't let you do this. Even if you could do this on a jailbroken phone, I could see it being abused by the wrong parties.
If all you are asking is how to dial a particular phone number from within your application, you can simply use the URL scheme like the following:
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"tel:1-555-555-5555"]];