So you mean "gateway" as VoIP gateway or "gateway" as a property of IP route? If VoIP gateway, you're not "getting IPs" from them in the sense of getting this assigned as your router's address by DHCP or PPP, but it means that they are expecting you to register from this address as source, and ignore registrations from any other address? Or it is reverse and 172.20.8.1 are the addresses of the registrar server, and you need to make sure that each of your two VoIP devices will be registering to the same remote IP (172.20.18.1) via a different route/WAN? Maybe a drawing will help...
there is no dhcp or ppp. the voip working with static route cable connected.
VOIP1 >>> Mikrotik Gateway:10.136.112.73, IP:172.20.8.1 >>> Elastix Server
VOIP2 >>> Mikrotik Gateway:10.134.160.1, IP:172.20.8.1 >>> Elastix Server
the problem is VOIP1 only registered with 10.136.112.73 gateway from 172.20.8.1 IP and can't be registering with 10.134.160.1
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