I assume the cellphone is running Android. Install a browser that supports WebRTC. Firefox does, probably Chrome too.
Go to
obitalk.com/obinet. No need to log in. Click the green 'Call OBi' button, complete the captcha, enter the 9 digit OBi* number and click 'Call'. If asked, give permission for the microphone to be used.
This only works for calling
to an OBi.
Or install the Obion for Android app and give it a softphone number. It can then call or be called just like any other OBi.
(except OBi1xx devices.)