you need to have the service running on all CMs if you want to provide redundancy in case.
You only need to subscribe the EM on the phones that need it.
Understanding Extension Mobility and Phone Logins
The Extension Mobility feature allows users to configure any Cisco IP Phone 7940 or Cisco IP Phone 7940 IP phone as their own, on a temporary basis, by logging in to that phone. Once a user logs in, the phone adopts the user individual user default device profile information, including line numbers, speed dials, services links, and other user-specific properties of a phone. For example, when user A occupies a desk and logs in to the phone, her directory number(s), services, speed dials, and other properties appear on that phone; but when user B uses the same desk at a different time, his information appears. The Extension Mobility feature dynamically configures a phone according to the current user.
Note If a login device profile is configured for a Cisco IP Phone 7960 but the user logs into a Cisco IP Phone 7940, the phone takes on the attributes and capabilities of the Cisco IP Phone 7940. This is because there are fewer lines on a Cisco IP Phone 7940 than a Cisco IP Phone 7960. The Cisco IP Phone 7960 capabilities return when the user logs into a Cisco IP Phone 7960.
Previously, only administrators could change phone settings only through Cisco CallManager Administration. The Extension Mobility feature allows users to change phone settings themselves without accessing Cisco CallManager Administration. Instead, when users authenticate themselves at the phone, a login service performs the administrative updates.
The programmable login service enforces a variety of uses, including duration limits on phone configuration (persistence) and authorization to log in to a particular phone. A Cisco IP phone XML service provides the user interface to the login service provided in this release. Refer to the Cisco CallManager Administration Guide for more information.
Supported Phones and Features
This release provides Extension Mobility feature availability on Cisco IP phones that support Cisco IP phone XML services; currently, only the Cisco IP Phone 7940 and the Cisco IP Phone 7960 offer this feature.
Managing Device Profiles
A device profile comprises the set of attributes (services and/or features) associated with a particular device. Device profiles include name, description, phone template, add-on modules, directory numbers, subscribed services, and speed-dial information. Two kinds of device profiles exist: autogenerated and user. You can assign the user device profile to a user, so that, when the user logs into a device, the user device profile you have assigned to that user loads onto that device as a default login device profile. Once a user device profile is loaded onto the phone, the phone picks up the attributes of that device profile.
You can also assign a user device profile to be the default logout device profile for a particular device. When a user logs out of a phone, for instance, the logout device profile loads onto the phone, giving that phone the attributes of the logout device profile. You can create, modify, or delete the user device profile in the Cisco CallManager Administration web pages.
