We've fixed the problem in making a call which was caused by the server failure from 7:40 pm on May 30 (JST).
Would you please try again if you found the problem?
We are afraid that the problem might continue if your OS version is old. We appreciate your understanding.

(Updated as of June 4, 2020 (JST))

We apologize for the inconvenience and thank you for your patience.

Update as of June 4, 2020 (JST):
We've fixed the problem which continued on some of the devices running an old OS version.
Would you please try making a call again if you found the problem before?