Whats the exact type of softphone you're using?
After a lot of installs, the best headset I have found so far that works seamlessly with most Mitel products is the Sennheiser SDW 50xx range.
In my case, this has been used for various MiCollab installations and the feedback has been excellent. They are clear, robust, modern and easy to use, they are also Mitel/UC certified.
There is some software which comes with the device called HeadSetup Pro from Sennheiser. Once this is installed it gives you the choice to choose your default softphone and also control the ringing volume and tone through the base station.
Its solved 2 major problems we've had with PC based headsets and softphones so far. It allows the ringing to come through the base station on an incoming call which saves messing round with Windows 10 audio settings and potentially buying a set of PC speakers. It also allows the user to control the call through the button on the side of the headset, this can allow them to answer and hang-up a call - This is all without conflicting with any other UC install on the machine, skype for example.
If you're just after a headset for use with PC, not a deskphone or mobile connection, the SDW 5013 would be suitable.
https://en-uk.sennheiser.com/sdw-5013Theyre pricier than your average headset but you get what you pay for these days..