All versions prior to 5.6.x enable SIP ALG by default. You have to put the "config.ugw.voip.sip_alg_disable=true" in config.properties.
Double check it's in the right place and being applied. SSH to USG and run "show configuration commands |match modules". Will see something like the following if it is.
$ show configuration commands |match modules set system conntrack modules sip disable
As long as you see the "sip disable" there, it's disabled. If that's the case, when you SSH in and disable it again, it's not actually changing anything, the only change in that case is rebooting the phones.
If disabling SIP ALG 'fixes' your issue, you need to create a custom JSON file, to make your CLI changes survive a re-provisioning of the USG. See here for a general guide:
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