"Robotic" sound problem

I installed FreePBX on Hyper-V. Everything is fine, but sometimes agent’s voice is heard “robotic”. This problem occurs in both external and internal (extension -> extension) calls.

An example recording: bit.ly/1BPD8YP
Asterisk full log of the call: bit.ly/15Du6AK
Asterisk SIP settings: bit.ly/1y77pef

What is the cause of the problem do you think? What should I check?

Asterisk 11.14.2
CentOS 6.6 x86_64 (installed on Hyper-V)

Agents use Eyebeam ( Build 30936) softphone and Jabra (UC 5550) headsets.

Possible bandwidth problem, huge latency issues… try checking that, otherwise I have no idea.