Spark IM audio call error

Hello everyone, I have a problem on my spark, where some audio calls present the following message: “The session Was Rejected” that even before establishing a connection, because the error is displayed when making the audio call. In other situations I have this error message "Session closed due to Unable to negotiate session.This may be caused by firewall configruation problems."

and the other side:voice chat ended:closed remotely

Does anyone have any idea of what may be this error?

Sorry for my poor english

Thanks