Can you give a quick over view on how SIP differs from HTTP in functionality and what are the problems of using PSTN as a back for VoIP?
SIP and HTTP are very similar in structure as they are both text based application layer protocols. HTTP allows for transferring hypertext based files between remote and local systems. SIP allows for setting up and tearing down sessions using any type of media (e.g., voice, video and fax). Many people today that are using VoIP service often have backup PSTN service, or use PSTN service in conjunction with their VoIP service. This is mainly for redundancy and also for toll bypass. There are no real problems with this as long as a proper VoIP phone system which supports these features is chosen.