It isn't required for the CallManagers to be in a domain, even if you have the directory integrated with your Active Directory domain. It is supported for them to join your domain. Joining them to a domain has no operational impact, but you should follow procedures carefully when performing upgrades, as some upgrades require you to dis-join the domain temporarily.
You cannot, however, make them domain controllers. This stops you from upgrading them or applying service packs, as doing this requires you to be logged in as local Administrator, which you cannot do on a domain controller.
Take extra care if you integrate with Active Directory and apply group policies to the CallManager server computer accounts. Some items, like password lockout settings, can get you into trouble. You will have no end of trouble repairing your cluster if your SQL service accounts get locked out, and this WILL happen during upgrades.
