And SMS/MMS texting doesn't enter into the equation in any way shape or form, because cell service is not enabled on my iPhone.
Phone number addressing of iMessages works only between Apple devices with an Apple ID. The actual routing is via email not the cellular network. However Apple is capable of injecting a text into the telco network when you are using a device such as a Mac computer that does not have a cellular connection.
I don't know if this is the same thing, but it's worth mentioning that every cellular provider has got email addresses via which SMS and MMS messages can be sent to/received from a computer.
The ones I know are...
Verizon: <@vtext.com> (SMS) & <@vzwpix.com>( MMS)
AT&T: <@txt.att.net> (SMS) & <@mms.att.net> (MMS)
