Here i am again and it seems that most of my questions or problems are either difficult to answer or unable to resolve.
But will still give this a try and see if anyone from you guys had experience this issue.
Skype to Skype call are all good and working. However, our environment got one special setup wherein the main number (which is a standard PSTN phone with handset) configured to forward calls to a Skype for Business number. when this setup is in effect, callers from outside when they called the main line and then the call being forwarded to Skype for Business number, the caller can not hear any ringback tone. it's either callers will just hear someone saying hello because the call has been pickup or it will go to voicemail (but no ringback tone). if we disable the call forwarding from main line to Skype for Business, everything worked perfectly. So what could be the issue? is it in the Skype for Business environment or is it from the callers Telephone Service Provider or is it in our gateway?