When installing the Cisco IP Communicator (soft phone) on your computer, if you are seeing that a dynamic assigned number from your CUCM installation, these numbers that you get are set in the Auto-registration fields under System > Cisco Unified CM > <select server>. There will be a range that someone has already setup and that range is used. Once you reach the end of this range, it will stop automatically assigning numbers and you will need to do it manually.
As per the Cisco Unified Communications Manager system guide, when you enable auto registration, you specify a range of directory numbers that Cisco Unified Communications Manager can assign to new phones as they connect to your network. As new phones connect to the network, Cisco Unified Communications Manager assigns the next available directory number in the specified range. After a directory number is assigned to an autoregistered phone, you can move the phone to a new location, and its directory number remains the same. If all the autoregistration directory numbers are consumed, no additional phones can autoregister with Cisco Unified Communications Manager.
Refer Cisco Unified Communications Manager System Guide: Autoregistration for more information.
