The max-ephones command within CUCME has been upgraded as of CUCME version 4.3 (and later). So, if you've had some experience with this command on some of the older CUCME versions, you will want to pay close attention to how this command works in the most recent release(s). This translates to roughly IOS version 12.4(15)XZ and later.
Cisco Unified CME and Cisco IOS Compatibility Matrix
Download CME Software (requires CCO account and authorized contract)
Prior to the CUCME 4.3 Release
Prior to Cisco Unified CME 4.3, the max-ephones command specified both the maximum number of SCCP phones that can be registered AND configured.
In Cisco Unified CME 4.3 and later versions, the max-ephones command is enhanced to set the maximum number of SCCP phones that can register to Cisco Unified CME, without limiting the number that can be configured.
Also, the number of SCCP phones that can be configured is no longer limited to the maximum number that can be registered. Also, it is not limited to the hardware platform. This change expands the maximum number of phones that can be configured to 1000.
However, the maximum number of phones that can register to CUCME is still dependent on the number of phones supported by the hardware platform and is limited by the max-ephones command.
This enhancement supports features such as Extension Assigner that require you to configure more phones than can register. For example, if you set the max-ephones command to 40 and configure 80 ephones, only 40 phones can register to Cisco Unified CME, one at a time in random order. The remaining 40 phones cannot register and an error message displays for each rejected phone. This enhancement also allows you to assign ephone tags that match the extension number of the phone, for extensions up to 1000. (Yes, I agree. That would be a fun router config to deal with.)
What Happens If I Change Max-Ephones?
If you reduce the value of the max-ephones command, currently registered phones are not forced to unregister until a reboot. If the number of registered phones, however, is already equal to or more than the max-ephones value, no additional phones can register to Cisco Unified CME. If you increase the value of the max-ephones command, the previously rejected ephones are able to register immediately until the new limit is reached.
What is Extension Assigner?
Extension Assigner is a feature that gives installation techs the ability to assign extension numbers without needing administrative access to the Cisco Unified CME, typically during the installation of new phones or the replacement of broken phones. However, before the feature can be used, the system administrator must first configure Cisco Unified CME to allow specific extensions to be assigned. The system administrator must also provide the installation technician with the information necessary for assigning extension numbers to phones. The installation technician can then assign extension numbers to phones with access to only the phones themselves and with no further intervention from the administrator.
Extension Assigner Details
Cisco Unified Communications Manager Express System Guide