Denphone activated the JUSA Tone on all calls using fixed line telephone numbers on the Cloud IP-PBX and SIP Termination services.
 

In Japan, following updates to the Telecommunication Law in 2019, all telephone services which assign fixed telephone numbers (such as Tokyo 03, Osaka 06) but do not exclusively traverse the traditional Public Switched Telephone Network (PSTN) and partially utilize the public internet and other networks (including cloud PBX services), due to the fact that in some cases they may not meet the same technical quality standards as a service run exclusively over the PSTN, a legal mandate was delivered to all telecommunications providers to insert a tone so that users are able to distinguish this difference. At Denphone, to comply with this mandate, we have chosen to insert the tone defined by the Japan Unified Communications Service Provider Association (JUSA).

Please refer to https://www.jusa.jp/tone for more details.