I am not sure what you are trying to do ,I am assuming you have connected three 831 cisco routers to the internet ,now you wish to do voip between these three sites .831 does not support Voice , check the following Links for routers which support voice,
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anthor option which you could try is Install an asterisk server at one of the locations behind one of these routers on public ip and register sip phones on the internet from the other sites .This would not involve buying more hardware if u use sipsoftphones.Also you could use ATA boxes behind these routers and configure them for differrent Voip providers like Vonage or FWD.
Thanks Aman