I think I found what the problem is. The customer site has VB-44210 phones and IIRC they have a speakerphone but no microphone. When their phone system was moved, they received new phone. The system was automatically muting the call because the handset was on-hook. I had the customer press the on/off button while talking to me (leaving the handset off-hook) and he was able to mute/unmute me.