What version of firewall code are you running? On R77.30 all VoIP traffic can only be inspected by the lead Firewall Worker core (fw_0 - usually the highest CPU number), if you have IPSec VPN traffic present it can only be processed on that same core as well for R77.30.
On R80.10 gateway IPSec VPN traffic can be processed on multiple Firewall Worker cores, but I don't recall any mention of VoIP inspection improvements in R80.10 gateway, so I assume the single-core VoIP inspection limitation still exists in that release. Edit: CoreXL known limitations (sk61701) states that the VoIP single-core limitation only applies in R77.30 and lower.
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