I know many people who don't want to take part in Mumble conferences because they can't use their phone there. So that team is locked into using something else (like Talkyoo), where you don't even see who is talking.
Adobe Connect supports logging in by web and then enter your phone number and let the system call you. Another possibility would be to call the server and enter some ID code.
Mumble could add this by logging into SIP accounts and accepting/initiating voice connections over that account.