ARS 6.8 Web Interface - need to attach custom JScript to auto format Telephone Number

We would like to be able to auto format Telephone Number on a fly in ARS 6.8 WI.

We used to be able to do it in the old good ARS v.5.2.x, by customizing ASP page - by attaching an "onchange" event that triggers cusotm JScript.

Is there anything we can do in ARS 6.8 WI to add similar functionality?