There are several ways of approaching this issue.
- First try to get a better understanding of whether the local network traffic is impacting the voice.. If all local network traffic is ceased, is the voice quality OK or is it generally always bad regardless of the local conditions?
- Do you have any router in place besides an ISP provided router? You may be able to get deeper access to the router config by contacting your provider.
- Depending on the model of SIP phone you are using, you may be able to set the QoS tags at the Ethernet level within the phone. This may or may not make a difference. It really depends on whether your network router will respect these tags. You should look for settings for 802.1Q in the phone and try setting the highest priority.
- At the IP level, you may want to look for Settings related to Type Of Service (TOS). Try setting these to high priority, and again, if your router respects TOS, it would prioritize.