skype Group call failing

Hi Leads,

Am facing an issue in the Skype since my clients are abroad only medium for the communication is Skype,the actual issue is when group call is initiated from various participants from various region it getting failed for me since am in a corporate environment using Skype is restricted any way privileged access to it can be gained so i got the access now after a long R&D we came to find the firewall is blocking the connection of incoming group chats so if we need to allow the access in firewall what IP should be enabled since Skype has multiple number of inbound and outbound IP addresses it is hard to track so is there an common or default IP address or port which can be allowed to avoid all this consequences hoping for the early response

thanks

Sharath