I chaired a conference call today on an AT&T service. As people were joining (typically about 20 attendees join this regularly scheduled call), we were suddenly treated to a high-pitched squeal (remember 300baud modem carrier tones? like that!!). After about a minute of all conversation being overwhelmed by this interference, it disappeared. We began our meeting and a minute later the squeal returned, but only for 20-30 seconds.
One of the attendees mentioned that this seemed to occur when someone dialed into AT&T calls using Skype. A moment later, one of the attendees confessed that he HAD joined the call using Skype.
This led to speculation that AT&T's conference service might be blocking Skype users by using this tactic.
I'd be interested to hear whether anyone else has had similar experiences with AT&T or other conference service providers.