Are you sure Sprint allows DUN on its phones? Verizon doesn't always you see. And, Palm phones are a bit of a special case and might require special modem strings to get them working. Also, check here:
http://www.internettablettalk.com/fo...hlight=Verizon
http://www.internettablettalk.com/fo...light=pap+chap

Finally, if this Treo is Windows Mobile based and not PalmOS based, you need to use the Bluetooth PAN third party application instead to connect.