dropped packets over VPN

Hi All, 

I have a network that spans 8 physical locations over a VPN, Sites 2, 3, 4, 5, 6 , 7 and 8 all have netgear firewalls but site 1 (head office) I have the sophos XG firewal. All locations have a panasonic digital hybrid phone system.

 

Previously site 1 also had a netgear firewall, I switched it out for various other reasons but my site to site VOIP over the VPN worked flawlessly. Calls to and from 2 to 3, or 2 to 5, 5 to 6 ect..anything that doesn't originate or terminate at site 1 still work perfectly, but on or about when I switched site 1 to the sophos firewall I started having call quality issues with any calls that do originate or terminate at site 1. (I am assuming dropped packages, the sounds cuts out momentarily, there's a hollow sound and i lose a word or two from the audio every few minutes), sometimes it's worse than others but at pretty much all times it makes it near impossible to carry on a conversation)

 

The ISP claims they have run every possible test and there is no packet loss or instability with my WAN. Could this be the firewalls fault? what settings could I adjust to attempt to rectify this? 

 

I appreciate your patience and detailed responses, i'm quite unfamiliar with the sophos product.