How to set up Asterisk on a Debian VDS, to which cSipSimple clients will connect via VPN?

Question

Asterisk

Good afternoon.
Most likely a similar question has already been asked somewhere, but I could not google it.
I would like to run asterisk on vds with debian. To connect to asterisk, only clients connected via vpn could. It is assumed that these will be androphones with csipsimple, in the asterisk there will be directmedia=on.
Is it possible to do so? Tell me, please, what manuals to read.

Answer 1

Raise VPN services
Three options at once are better:
ipsec - for Android and iPhone
ppptp - for old computers
OpenVPN - for those who do not like breaks
Set FreePBX
Prescribe settings
In general, if your hands stick out from where you need to work for 6-12 hours