I use an 877 number all the time, I guess I could try a local number. Either way, I suspect the conference line is just sensitive. But sensitive to what? If this was a problem on their end, I'm sure they would have it resolved by now. It must be hearing double tones (or none in your case), so where is the problem originating?

I'm leaning toward thinking it is the VOIPo server I connect to, although I wouldn't immediately rule out DD-WRT. More research required!