IPsec VPN is a standard protocol that allows a variety of solutions for endpoint connectivity, including FortiClient.
It is a well defined protocol that uses specific ports, and it is not uncommon for ISPs to block these ports. On the FortiGate, administrators can configure the ports used for IKE (UDP 500 and 4500) (see Configurable IKE ports). IPsec also has the option to accept a peer ID to specify a tunnel if several tunnels exist on the same interface.
For more information, see IPsec VPNs in the FortiOS Administration Guide.