On the change event of the area code field you can use javascript code like this:
var tempStr = xfa.event.newText
if (tempStr.length == 3){
xfa.host.setFocus("TextField2")
}
The tempStr variable wil contain the input that the user selects. Assuming that the are code is 3 chars then when they enter 3 chars the the focus will change the the named field in the setFocus command (in my case TextField2).
Paul