When you buy 2821 router it will be most likely version with IP-base software and no modules. You need IOS version where you have CCME included and probably module card for PSTN connection (E1 or FXO card depends what you have there).
Best option is to talk about your requiremnts to your local Cisco partner.
I have downloaded CCNA Voice book where it is stated that we should buy 3 Licenses ( IOS,Feature&Phone) from Cisco to run our mini VoIP solution and in that way We get the CME software(may be download it from cisco.com) and copy it to the Router flash by the aid of TFTP server and finally extract it on router's flash and then start configuration.
So this basically will fit for intra VoIP solution where users can call inside their company without calling outside.
Now for external call , we need a modular card to be added.
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