Unless you have a specific need for functions that only offered in VOIP phones, you should not go to VOIP for local telephony as a change .
For international calling, is a must. USed together with your actual PBX equipment, you do not change anything on the phones, bur stop feeding the long distance companies.
For long distance, depends where you are at and how expensive the long distance calls are.
In some places, you still need to pay close to a one dollar a minute to talk across the border, only 50 miles away.
Getting a gateway that interface with your PBX at a E1 or T1 level will guaranty high quality audio.
The best way to start, will be routing all international calls out of the main PBX to a gateway to a VOIP provider.
That setup normally pays for itself in months.