On demand VPN for voip

Hello,

I need to remote users, connect to main office from where they are, so they will connect from different places, so I need an on demand VPN.  They will use a softphone in their Windows computers

My knowledge about voip is very poor, but I think that the communication will work in both ways (external devices to internal network and internal network to external devices)

Also is needed that remote users can access internet through their local gateways (split tunnel)

In the main office I have a Fortigate 60C with firmware version 5.2.11

I tried with many different VPNs but I have different issues with all of them.

Last time, I have created a forticlient vPN, I got the remote users connect to the VPN and navigate through their default gateways, also remote users could access to the local network, I could ping from external computers to internal servers but I could NOT ping from internal servers to external computers.

Anyone could make it work?

I could make it work everything with a mikrotik router, so I tried to use this as internal VPN server behind the fortigate but I could not make it work behind the Fortigate 60C 

I cant install a fortigate in remote places because is not allways the same place.

Any help will be apretiated

Regards,

Damián