Your broadband speed may be sufficient without QoS settings, depending on your other streaming activities.
I've read one VoIP channel/session can require ~87Kbps or less in one direction depending on the codec used. You can set aside bandwidth accordingly, if necessary.
Hopefully your router NAT will not be an issue. Do disable any router SIP ALG or SIP Passthrough function, as it typically messes with SIP traffic.
OE