Solution:
These attributes are required to be returned by the RADIUS server:
Service-Type = Callback-Framed
Framed-Protocol = PPP
Callback-Number = xxxxxxxxxxxx
If the Callback-Number value is blank the Perle will prompt the client for a callback number (Roaming Callback).
Note: with Windows 2000 IAS the Callback-Number configuration is done within the Local/Active Directory Users and Groups. See the Dial-In tab of the user profile. Windows Server 2003 uses the Attribute list.
Note: If the user also requires to be an Administrator then you must configure the Service-Type = Callback Administrative.