I always disable SIP ALG on my router even if my phones seem to register ok, It appears that disabling SIP ALG now prevents any of my phones from registering with their VoIP service providers!
I had to select "Enabled" or "Enabled + NAT helper" located under the WAN > NAT Passthrough page to get my phones working again. The default value is "Enabled + NAT helper" so unless you changed it or your having issues registering or making calls don't change it.
If your wondering what the difference between "Enabled" and "Enabled + NAT helper" is - If you select "Enabled + NAT helper" then Linux's Netfilter will try to detect when something is a SIP connection, and when it does, it will try to automatically handle inbound traffic and bypassing the firewall. Some provider might need it, others don't as they have other methods of traversing through NAT.
Never thought I would be telling anyone to enable SIP ALG!!!