How can i connect random servers with asterisk

I am creating VOIP environment using Asterisk, well I want in this environment that user should not connect to the Asterisk Server direct; For example I have one main server Asterisk in INDIA and 3 other servers are hosted in US, UK and Australia then two users wants to communicate BOB and ALEX using VOIP service so BOB should connect to the random server in US and ALEX should connect to the Australia then both server should connect to the Main Server Asterisk which hosted in India.
I am Upload a screen shot for this model.

Here is the image
Please help me how can I implement this, is there any possible way with VPN is so please guide me.

Thank You

Why are you trying to do this? Can you clarify the meaning of “random”?

If you can do it with Asterisk, sever1 and 2 will also have to be Asterisk, but I suspect you may actually need to use SIP proxy servers.

For future reference support questions belong in forums with Support in their name.