Mint2K

Hi, Has anyone had any luck getting Office 365 to work with on premise PSTN using a SIP trunk from Gamma? We are experiencing issues with 403 forbidden errors and Gamma are not being very helpful. CCE is configured as per Microsoft's guidance. Traffic is confirmed to be getting to Gamma through our firewall, but then rejected due to private IP being included in the SIP header as part of the VIA address. Any help would be appreciated. Thanks Trev