A single call is going to use very little bandwidth (if G711, roughly 64Kbps), so it totally depends on the number of concurrent calls you will have on that network at any given time. That being said, if the number of concurrent calls is reasonable then you should just go into your router/firewall and give the phones priority over the bandwidth using QoS and BWM so that if your circuit is being fully utilized you won’t suffer from call quality issues.
That being said, call quality also fully depends on your ISP which is a whole different matter.