Is the SIP functionality really gone for good from the Optimus 4X?
Actually, LG really seems to think their (European?) customers don't need SIP. Most likely official argument will be that carriers either exclude SIP usage from their mobile data plans (at least here in Germany, many do). So they removed it.
Ooops. That was only half true: They removed it from the obvious places, that is. You might not find it in the settings. But luckily, they just removed the link to the settings screen -- all the code still seems to be there!
Ah! Still available! So how to access it?
I found it more accidentally while playing with Apex Launcher. Long pressing on a free space on one of the home screens, like you want to add a widget, opens the expected menu. Select "Shortcuts" here. On top of the next screen you'll find an item called "Activities", which I long ignored -- unfortunately! But after a very interesting chat session with Dan on broadcasts versus intents versus activities, I had to dig into this. And found that:
activities of the phone app (click images for larger variants)
The top entry in the first screenshot clearly reads Sip Settings -- so I added that shortcut to my homescreen. And what should I say? Yesss! It's the long missed settings screen, from where you can configure your SIP accounts (and define whether you want the device to listen for incoming SIP calls)!
So if you miss something that should be there, this might be a place to look for it. Besides, as reputated users as ASE we're always told to watch out for hidden gems in new places -- so you might want to look further:
list of apps with activities (click image for larger variant)
You see, other apps have also activities they made freely available (they might have even more which are kind of "private" to them). The numbers to the right tell you how much activities the corresponding app has made "public". Try them out! You surely will encounter the one or other force-close when you try to start such an activity (as it might expect parameters), but it cannot hurt. You might find other interesting things!
Are there other ways to discover/call such activities?
Sure, Apex is not the only one. There are other apps available on the playstore which let you explore available activities, such as e.g. Package Explorer or Stanley. And you also can use e.g. Tasker to start your discovered activities.
Besides: With that, most of the "shortcut-apps" available on the playstore you won't need any longer this way. Just inspect the Settings app, which seems to declare each and every page it has -- so you can directly jump there from your newly created shortcut!
Does this apply to the Optimus 4X only?
For sure not! This functionality should be available on any device. I just cannot promise you to find the SIP settings on all of them. Or anything else you've seen in above screenshots :)