connect Kamalio Db to A2billingn over OpenVpn(50 - 100)

Our Setup is like this A2billing handles Account stup, Public Kamalio handles users on mobile internet, remote local Kamailios handle users over wifi. we need connect local Kamalio Servers to A2billing server Via OpenVpn for user registration. You must know Kamailio , A2billing , Mysql and OpenVpn . Kamalio Servers setup is already done. A2billing is already done all working just need to connect Database of local to Remote A2billing server.