I am not sure I completely undertstand your requirement.
> What do you mean by filter out VoIP traffic? You want the traffic to be blocked/permitted/not-firewalled?
> Could you explain the loopback issue? What needs to be looped?
> What is the topology like? Cable modem ---- SRX ---- Internet
> You mentioned cable modem is working on the SRX and VOIP traffic is passing through as well. Correct?
> Do you have the IP address of the modem?
> Session output from the SRX for the source of the modem could provide more insight into the type of traffic?
> SRX would be doing the NAT for the modem traffic as well right? If this is the case you cannot bypass firewalling
Im afraid that your problem is still unclear to us. Is your topology like the following one?
Are you looking to prioritize traffic from the VOIP_Modem to the Internet?
Are you looking to block the traffic from the VOIP_Modem to the Internet?
Are you looking to bypass firewalling for traffic from the VOIP_Modem to the Internet?
Does the SRX has to perform NAT to the traffic from the VOIP_Modem to the Internet?
What do you mean with loopbacks?
If you are looking to block phone calls from the Phones to the Internet via the SRX, you could confirm whats the signaling protocol being used and block this traffic via a security-policy or with a frewall filter. For instance, a very common signaling procotol is SIP, which works over TCP and UDP on ports 5060 and 5061. Blocking this type of traffic via a security-policy or a firewall filter should be enough to avoid the calls to be made over the SRX.