I disagree, getting a SIP server up and running in EC2 is no harder than doing it on it's own. Unless you're doing mixing/conferencing, you shouldn't see any delays from the virtualization.
This is especially true if you run FreeSWITCH in bypass media mode. If all you're processing is the SIP signaling, you can put a lot more calls on a virtual server AND usually see better quality calls as the RTP is typically sent on a more direct route to/from the carrier to the endpoint.
