[29083011397778]

Hell, even Android, which had built-in VOIP support in the default AOSP dialer app, had VOIP support ripped out. It now requires a third party app.

[Nextgrid]

Since LTE and IMS, all phones internally have a built-in SIP client already. If you're on LTE or using Wi-Fi calling, you're using SIP under the hood. There is no reason that client can't be opened to allow people to add arbitrary SIP accounts if it wasn't to protect the carriers' monopolies.