VOIP depends on your connection, sound card, OS, provider.
Leased lines depends upon the equipment being used by the company, weather conditions, other radiation- electric storms, aurora borealis, etc.

If you have a goods OS and good equipment deicated to PBX- even a personal one- you will get good sound quality.

You can:
Both have the pbx setup, a good OS, good equipment, and connect.
Good leased line providers and connect.
Find a time where the traffic is less for you and the conditions are optimum for him.