There are two possible explanations. Either your own MSN (here: YYY) is not correctly set with "isdnctrl eaz interface YYY". Or "isdnctrl secure interface on" was set, without allowing calls from the incoming number (here: XXX) with "isdnctrl addphone interface in XXX".
The Sun tries to communicate with asyncPPP. ipppd can't handle this, you have to use the ttyI* devices and the standard pppd.