Thanks for the screenshot. It looks like the call is in voicemail mode, so the keypad and mute button are intentionally disabled. There is a bug in telephony-service that could set conferences as voicemail calls when the voicemail number isn't provided by the sim card, which explains the problem.
-- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to telephony-service in Ubuntu. https://bugs.launchpad.net/bugs/1398431 Title: Can't mute a conference call Status in the base for Ubuntu mobile products: Confirmed Status in Telephony Service: New Status in telephony-service package in Ubuntu: Confirmed Status in telephony-service package in Ubuntu RTM: New Bug description: [Procedures] 1.make a conference call 2.in calling interface click the mute button [Expect results] the conference call will be mute [Actual results] the mute button does not respond and audio is still heard by others on the call [Reproduce] Always To manage notifications about this bug go to: https://bugs.launchpad.net/canonical-devices-system-image/+bug/1398431/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
