Best practice for load 2 wan(load balanced) +1 wan for voip

Hi

I've got a utm with inside 3 LAN and 3 wan connections

My 3 LANs are

internal

VoIP

Wifi

Outside I've got 3 wan

2 cable (A+B)

1 vdsl  C

My goal is to use

A+B (load balanced ) for internal and wifi traffic to internet

C dedicated to voip traffic to internet (my ipbx is cloud based so outside the utm)

I created load balancing and balancing rules for A&B

Masquerading rules :

voip => C

Internal group (internal & wifi) => uplink interfaces

It seems that it's working fine but I noticed that few iPhone on voip network are registered in the cloud ipbx with B outside address

Must I had other rules or route or else to force voip network only go out only through C wan ?

Thanks for your ideas

Fabrice