Need a lot more info to help.
- What up/down speeds do you have on your internet connection (speedtest.net or similar)?
- Is it a single connection or do you have a multi-WAN router?
- What router make/model do you have?
- What QoS settings do you have in the router?
- Are you confident the problems are not within your own LAN?
- Do you have SIP ALG switched OFF in the router?
- How many simultaneous internal/external active calls?
- How many phones? Makes/models?
- Do the call problems occur with specific phones only?
- What switches do you have between the phones and the router?
- What codec are you using?
There’s very little “magic” about QoS - it’s really just a “weighting” system to determine which packets get preference IF AND WHEN the pipe gets busy. So the first task is to know exactly how busy your connection is getting - a good router should have the tools built in to help you with that. My Draytek 2960’s also show me dynamic bar charts of low/normal/medium/high priority traffic, as a way of verifying that the QoS settings are appropriate.
Depending on your non-VoIP traffic, it may be worth installing a second internet connection dedicated to voice traffic, using a dual-WAN router to direct the traffic.
J