We have are having multi customers who have issues making more than 3 calls. After 3 calls call quality goes into really bad jitter. We have even experienced it for ourselves. We have an 8 call 3cx system running as an Hyper-V VM 4 CPU 4GB RAM 100GB HD space on a dedicate 1GB nic, using a SOPHOS Firewall in a Co-location where we have dedicated 100mb ethernet. We have 10 Extensions in our office and 4 remote extensions, however we have see the jitter start to happen when a call comes into the Ring Group. Our office has 25mb up/down with FIOS and we use about 2mbps at max. I am hoping (reaching) someone can help us understand if we need to change codec's or something to resolve this issue. We are using Yealink T28p phones.