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.