I've always had this problem with Skype calling on giffgaff. As if the traffic was getting deliberately stalled during the call resulting in the other side being able to hear me and myself listening to nothing bur silence. Funny that once routed via VPN, Skype calls were "all go", same giffgaff network, same location, etc.
So, being in Greece now, I've decided to test it once again.
Got EE sim, got giffgaff sim, both roaming on Wind, both on 4G. Speedtest is showing about 30mbps on each sim with ping around 100ms on both, give or take.
And it's the very exact same. Skype calls stall on giffgaff, yet work fine on EE.
The only difference were APNs through which all the traffic went - giffgaff's and EE's respectively.
Hence my suspicion with giffgaff (or, perhaps, O2 for certain types of users) being not the best friends with certain types of traffic.