Have not had this experience but also not using Jabber to the extent you are with a call center. Are these users external going through a Netscaler gateway or internal to their VM’s with a direct brokered connection?
Have you validated with Cisco the network port requirements for Jabber if the thin clients need to connect directly out the firewall to Cisco cloud infrastructure?
I’ve engineered our Zoom and WebEx for VDI environment, all these things have network port requirements to cloud data centers of some sort and our thin clients aren’t leveraging a proxy so you may have to punch some holes in the firewall for that.
I know Jabber has similar requirements, we’re still transitioning from 12.8 to 14 on the client side but the 14.0.2 JVDI plugin for the thin client side still works for us with the 12.8 client and agent on the VM side.
If all of what I said makes sense I would also work with your network team if they can do any on-site captures and analyze the traffic to see if anything is getting dropped or blocked our out of order.
Other thoughts:
Is your Citrix environment setup to use UDP, TCP or EDT?
What version of Citrix VDA is on the VM’s; LTSR or CR?
Are these standardized peripherals for your headsets and/or webcams?
Assuming your policy is NOT allowing USB redirect for audio and video devices, that’s required for the plugins to offload audio/video to the local thin client?