VoIP technology still has a few shortcomings that have led some to believe that it is not ready for widespread deployment.

One drawback is the inability to send faxes due to software and networking constraints. But now some VoIP gateways have started adding fax support.
Another drawback is the inability to make phone calls during a power outage. Re-wiring is necessary to use the phone jacks in a typical house. If VOIP is used in solitary LAN (with no internet connection), it would consume more resources compared to a PABX.