Title says it, "How do I adjust volume up for Zoiper 5 for ringing incoming calls ?" weird, as it is kinda low... it isn't comparable to other sounds on my system... I cannot adjust in sound mixer...
You can try right clicking with your mouse on the volume icon in the windows tray-> select "Volume mixer", from there you can adjust separately the volume from Zoiper compared to your other media.
As I said previously, in sound mixer, Zoiper isn't there... I said, I cannot adjust via sound mixer.
You can also change the volume by going to Zoiper -> Settings -> Audio -> click on the buttons with arrows next to the audio-device drop-downs. A pop-up with volume slider will open. Boost it to the maximum for "Ringing device" and if you want for the "Output" device.
While there's an Incoming Call it will be in the Volume Mixer.
So call yourself and adjust it then. See this screen recording: https://i.imgur.com/mWAjA6r.gifv