Under the WCAG Success Criterion 1.4.2: Audio Control, audio content is only required to have audio controls present when audio exists on a page for longer than 3 seconds. This means that if the notification sound used (a "ringing phone" in this case) persists for more than 3 seconds, users need to have controls to either pause or stop the audio or adjust the volume level (including down to 0).
However, if the audio does not last for 3 seconds, it is technically not required to have any controls. We would still suggest adding the option to mute the audio, as it would be considered best practice. For some users, the ringing could cause cognitive overload or overpower specific accessible technologies (such as a screen reader), resulting in a poor user experience.
For a step-by-step process to review accessibility considerations for audio, video, and other time-based media content, check out the Guided Manual WCAG Audit Tool from Accessible Web.