Call Lag issue with CX600

Hi, 

I am having the call lag issue with all CX 600 phone while making a call. vx310 and vx600 is working fine. 

The phone fimware is already updated to lastest version 4.0.7577.4531(April 2017). 

Phone netowrk range also added to persisten route of edge server. 

Is there anything i should check for the issue?

Thank in advance for the advice and suggestions. 

Best Regards,
Marry