Hi hopeng. It looks like Viber changes their call-API a lot the last few years. It seems that they have changed their API this year again... So I think that's the problem.
We'll need to test and investigate their new (and old) API to see what works the best and update our button-action if needed.
I have added it to our to-do list and I will notify you as soon we have an update for you 🙂