Waiting for Acknowledgement problem with SIP Phone

Hi,

My users are experiencing something weird with the 1.65.377a version of SjPhone. When ever they use the Internet the client goes into a waiting for Acknowledgment state, what would cause this and is this behavior that is set at the client or on Asterisk itself . I am not having the problems with Sjphone phones connecting from a remote site via a VPN tunnel.

Thanks