The reason your colleague said that the only way is to use XML, is that there is a WML (wireless Markup Language) defined. WML is a set of XML tags that is defined for wireless devices. The meaning of those tags, and how they shall be processed is well defined.

Regardless whether your colleague's comment is 100% acurate, following a well defined standard is generally a good practice. Your application in WML (XML) will be supported by lots of wireless devices from the day it is born with less effort on your side.