In July, 2012 a change was made to the Meetings service that prevents users from dialing non-US/Canadian numbers. This was done to increase call quality.
To get around this issue simple request a toll-free number from your conference provider to dial Meetings into. Since this number is toll-free it should not affect participants who wish to dial into the bridge and not simple listen to the audiocast.
Most conference bridge providers should have toll-free US or Canadian numbers.