I'm on Verizon, and my callers hear, “The party you called is not answering, please hold while your call is forwarded.”

Your carrier is Verizon and recently, every time someone calls to leave a message, they hear this, 

“The party you called is not answering, please hold while your call is forwarded.”

This is happening due to an update to Verizon's Ringback Tone system. Once the ringback tone is enabled with the call forwarding announcement, the call forwarding announcement cannot be turned off even if the ringback tone is inactivated.

This isn't an issue caused because you're forwarding to YouMail either. No matter what number you forward your calls to, this message will play. It's called a Ringback Tone and at the moment, there's no way to turn it off unless you're forwarding to the Verizon voicemail system.

Your best course of action is to contact Verizon and ask them to turn off this feature. If enough people report the problem, they'll focus some energy on the issue and have it resolved soon. YouMail would be happy to help fix it, but as the call forwarding is controlled by Verizon, they unfortunately hold all the cards in this situation.