This attribute specifies the user data to signal in a call request
packet. A value of null (empty string) indicates that the
callUserData from the X25Dte is to be signalled. If callUserData of
the X25Dte component is also null and protocolEncType of this
Peer has the value IP, H.CC will be signalled as the call user data of
a call request packet. Thus for IP encapsulation according to
RFC1356, both the callUserData from the Peer and X25Dte
components must be null and the protocolEncType set to IP.