⏰ In A Nutshell
"Mute During Mission" feature may be activated.
⏰ To Elaborate
If "Mute During Mission" is turned on, the ringtone doesn't come out for a certain amount of time you've set during the mission.
You can make changes in [Settings tab > Mission > Mute During Mission].
⏰ Keep This In Mind
If you want the sound to come out while you're on the mission, turn the feature off!