There is a log, which shows that registering is successful, but doesn't make reference to incoming rings. It works properly when IVM is not loaded.
I like your earlier suggestion, that IVM is 'taking over' the VOIP line from the phones, so that they lose SIP registration. If that were happening, what could be done about it? Can IVM be set to take over only after the right number of rings? Or is there something that could be done in the Belkin to allow both to run together?