CTIOS connection issue

I am trying to connect CTIOS server through my code. Everything is working fine in our lab but when moving to client site we are getting below error:

OnEventReceived Event: eOnConnectionFailure(ReasonCode:2 FailedServer:<<Side A IP>> ErrorMessage:CtiOsSession: Cannot connect to a server.)

OnEventReceived Event: eOnConnectionFailure(ReasonCode:3 FailedServer:<<Side B IP>> ErrorMessage:CtiOsSession: Cannot connect to a server.)

Below are some of the things I have already tried:

1. I already have put entries of Side A & B in the host file but no effect
2. Updated the dll file NetCil.dll & NetUtil.dll with correct version

I was not able to run any of the samples as it is windows 2012 machine

Please suggest, what can cause this issue

Since you are running samples, and the error you are getting is a connection error. Could you please verify that there could be no networking issues?