[otrs] Anyone know how to make a field mandatory?

I'd like to make the phone # field mandatory in the Add Customer screen of the Admin section. I need to do this because there is a customer requirement that we must record the phone # of the caller for all tickets that are called in. Thanks in advance. Daniel CONFIDENTIALITY NOTICE: The contents of this email are confidential and for the exclusive use of the intended recipient. If you receive this email in error, please delete it from your system immediately and notify us either by email, telephone or fax. You should not copy, forward, or otherwise disclose the content of the email.