Hi John,
This licensing is "honor based" and is not loaded on the router itself
Here is a clip that speaks to this question;
Prerequisites for Configuring Cisco Unified CME
Base Cisco Unified CME feature license and phone user licenses that entitle you to use Cisco Unified CME are **purchased.
From this CME doc;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmeover.html#wp2286954
The 3745 is an "older" model so you won't find much info still floating around for it. The 3745 will support 120 IP Phones.
Hope this helps!
Rob
Please support CSC Helps Haiti
https://supportforums.cisco.com/docs/DOC-8895
https://supportforums.cisco.com/docs/DOC-8727