Hi Alicia,
Here is the info for ISR G1 routers like the 3825
Applications and Migration to Cisco Unified SRST
Customers who initially deploy or purchase a Cisco Unified CallManager Express (CME) solution, the CME Feature License includes a provision to use the feature license for Cisco Unified Survivable Remote Site Telephony (SRST) mode without an additional charge or re-licensing to Cisco. Customers only need to re-configure the router for Cisco Unified SRST mode.
For the ISR G2 versions, I believe there is a $9 per user conversion charge now.
Cheers!
Rob
"And if I should fall behind
Wait for me" - Springsteen