Alcatel Unleashed

I currently have leased link from my ISP , this has VLAN 50 (company x ;private network configured by ISP to 3 sites and back to vlan 50 , sub net 50 oxo4 ) and VLAN 3 (public-internet)

I have connected this cable to port 1/1 of my switch , tagged vlan 3 & 50

I have added tagged port vlan 20, 21, 22, on 1/1 too

I have three oxos connected onto ports:

Oxo1 -1/20 ass vlan 20
Oxo2 -1/21 ass vlan 21
Oxo3 -1/22 ass vlan 22

There is a firewall in the setup and everything is ok with oxo 1,2 &3 for external connections

My problem

I have added

Oxo4 - 1/23 ass vlan 50

Oxo4 - communicates to the remote sites via vlan 50 and connects the remote sites phones back to CPU .

Now to make external calls OXO4 needs access to vlan3 for sip calls.

How can I do this ,, routing of some nature ?

OXO4 CPU cannot be part of two associated vlans ... Therefore how does specific traffic ie SIP calls route to vlan 3 (public ...to SIP host) ?

Thank you in advance.