Like every technology and services there is always a hidden problem that creates space for limitations
Mobility: Voice Over Internet Protocol facility can be effectively used only with the presence of an active internet connection in the location of operation. The reliance over the requirement of internet disturbs the continuation of a business process. Likewise there could be problems of power shortages while battery backups can provide a limited service. While with the PSTN you do not require power. Particularly here, organizations have experienced forced expenditure over effective battery backups.
Facsimile: Organizations as of today still use the usual telephones to exchange fax messages, VoIP does not have the technology as yet. However T.38 is an audio codec designed for Faxing over VOIP, it allows for more detail and better looking images as well as stability by creating redundancy in the RTP stream. Training on usage is underway hence appears as yet to be a drawback.
Safety and transmission issues: Technology has developed to be an active coordinator in every aspect of business today. However VoIP is currently experiencing improvement stages with regards to Latency, users face voice echoes, jittering and sometimes loss of quality of service from the service provider. Like how encryption is important while utilizing internet services, subscribers are yet accept that VoIP needs encryption for security purposes.
Other disadvantages of using Voice Over Internet Protocol are unavailability of options such as emergency calling, call tracing facility of few calls, example an emergency call to the hospital or police, receiver cannot trace the location or number of the caller.
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