IP phones can be simple software-based softphones or purpose-built hardware devices that appear much like an ordinary telephone or a cordless phone. Traditional PSTN phones are used as VoIP phones with analog telephone adapters (ATA).
VoIP is purely a cost saving exercise and is not reliant on IP Telephony to work.
Converged Telecoms provides SIP gateways to connect to legacy telephony platforms to VoIP enable these older systems. They continue to run analogue or digital instruments to the desktop. IP telephones on the desk are not necessary to enable costs savings linked to Voice over IP.