Hi Urooj,
I thought that I might send some info that explains the differences between Callmanager and Callmanager Express as it can be confusing sometimes. The other thing to note is that the word "Unified" was added to the product name a while back and now has changed again to Cisco Unified Communications Manager and Cisco Unified Communications Manager Express. These are just name changes and NOT different products.
Q. How does Cisco Unified CallManager Express differ from Cisco Unified CallManager?
A. Designed for small offices, Cisco Unified CallManager Express can support as many as 240 phones as a fully integrated solution in the Cisco access router. Cisco Unified CallManager Express and Cisco Unified CallManager use the same Cisco Unified IP phones, Cisco Catalyst? switches with inline power and quality of service (QoS), and the same router-based gateways to the PSTN. They offer many of the same features, but they also have some important differences. Cisco Unified CallManager Express is designed expressly for small-office deployments such as autonomous branch offices, retail stores, and service provider-managed services.
Designed for medium to large-sized offices, Cisco Unified CallManager can support tens of thousands of phones at one or many sites using a centralized call-processing design. Cisco Unified CallManager is a Cisco media convergence server (MCS)-based solution that can be deployed in one or more clusters for high availability. Cisco Unified CallManager is required for customers who want advanced applications such as Cisco Unified Contact Center, advanced conferencing needs, or extension mobility.
One of the many advantages of using Cisco Unified CallManager Express is that it can be deployed at multiple offices without the need to use the WAN for interoffice or voicemail calls. In the beginning, customers can depend on the PSTN to handle all interoffice calls and then can migrate voice onto the WAN as needed. Cisco Unified CallManager Express also fully supports a uniformed dialing plan between offices.
http://www.cisco.com/en/US/partner/products/sw/voicesw/ps4625/products_qanda_item09186a00801c637d.shtml
For CME the number of supported phones is based on the Router platform and Memory (like you have correctly noted). You will need to purchase a new box to increase the number of supported phones :(
For example: Here is the number for a 2811,2821 and 2851 with CME;
Cisco 2811
Maximum Total IP Phones 36
Cisco 2821
Maximum Total IP Phones 48
Cisco 2851
Maximum Total IP Phones 96
Hope this helps!
Rob
