I've not had chance to try it yet but looking at the v4.70 feature guide if you don't use multicast paging requires one MGI channel for each phone in the page group (so you may be running out of MGI channels), whereas using multicast only requires one MGI channel for the whole group.

Only works with OS v4.60 onwards, MGI16/64 v1.27 onwards, OAS v2.03 onwards, and SMT-i2200/2205/3100/5210/5220/5230/5240/5243/7130 phones.

DM 4.1.3 and 2.7.1 look straightforward, in DM 5.2.25 you list the IP addresses of the phones, or .255 for the subnet.

I've not used Ubigate switches, it depends if the pass or block multicast (which is a form of broadcast) by default.

What isn't stated in the guide is that multicasting doesn't play well in routed IP networks.