Anyone have experience with getting a managed phone SLA (Shared line appearance) on an unmanaged phone?

Currently we have 2 vvx410 phones that are sharing a line which is working great. However we are in need of getting that same treatment to an unmanaged phone. It is a non-polycom phone because we had to have a VPN phone. IA does not really mix and match shared appearances between unmanaged and managed. So looking for some help or suggestions.