Looking at the CCM 4.2(3) documentation I can't see any mention of limitations.
The only workaround I could suggest is to add the phones to a Line Group and make the current Shared Line DN a Hunt-Pilot. You could then use the broadcast distribution algorithm so all phones would ring at once.
Unfortunately if they definately require shared lines, I think your best option here would be to contact TAC.
Sorry I couldn't be of more assistance, but if it has helped please rate this post.
I remeber reading about a bug that did impact Shared Lines if there were more than 12 phones sharing the DN. I can't seem to locate it anymore :( Roberto, can you tell us what IP Phone model these are, what Firmware and if any 7914's are in play?
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.