A softphone which work properly

I use FreePBX 13.0.195.12 with X-Lite and MicroSIP.
From time to time I receive users report of number not found display on the softphone UI. When I investigate, I found out the real reason is because of the destination number was busy.
The Busy Here respond was received from the trunk provider.
Asking around and I found out that the respond codes can be interpreted differently from client to client (whether it’s a softphone or hardware phone).
I research further to find the best compatible softphone and found Zulu.
They say it’s developed by the same time who developed FreePBX. So I think this must be the best shot.
I researched further and found that they are saying

In certain networks, it’s possible there may be a small audio delay (typically 3-4 seconds) from the time you answer an inbound call until the caller can hear you

Ref.: https://wiki.freepbx.org/display/ZU/Zulu+3
This is the exact same behavior I am experiencing when using X-Lite.

Then I am thinking this might not be any better than the softphone I am using.

My setup:
The FreePBX server is located in a datacenter.
User connect in using laptop base VPN client.
Register their softphone to the FreePBX server within the VPN stream.

My experience with the current setup:
Voice is being cut-off intermittently.

Questions:

  1. Does anyone know a softphone which property relay the respond code?
  2. Had any one been through the same setup? If yes, what did you do you resolve the voice quality issue?

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