On Watchguard:
Create new Mobile VPN with IPSec (VPN -> Mobile VPN -> IPSec):
Put Group Name,
Put Passphrare, Phase1 (SHA1-3DES-DH2), Nat Traversal, Dead Peer Detection,
Phase2 (ESP-SHA1-AES)
Virtual IP address pool (IP addresses for mobile users)
Create User to have access to the VPN.
On iPhone: