Dual wan issues

Good day,
i have more than 2 lans all connected to my dual wan openwrt router.
my openwrt i configured with mwan3 with 2 isp.
ISP A a uses public ip which is configured with a lower metric
ISP B uses dhcp with a higher metric,
currently ISP B is the preferred gateway because its faster and more reliable. i have to manually change the configuration to use ISP B by lowering the metric..i can live with this,
my problem is i have an freepbx server running on my lan and only reachable from outside via my public ip on ISP A

how do i allow freepbx external clients to be able to use ISP A for incoming and outgoing traffic.
what is currently happening is that when the external clients (sip phones) try to connect from outside the request reaches my router via ISP A but the router is sending packet to the internet via ISP B which makes it impossible to for the external clients to register.
how do i use mwan3 to achieve this.
simply put i want sip traffic to use only a certain isp for connection, how can this be achieved

bind freepbx only to the wan port of ISP A ?

Hello Frollic,
your reply sounds more like a question
kindly elaborate on how i can bind the freepbx to ISP.
is this achieved via the openwrt or the freepbx gui

No idea how freepbx works, but I would assume it's done there, not in openwrt.