Based on comments and answers, there seems to be some misconception. Allow me to clarify:
- I have of course searched for REG-RESP on Google prior to writing this question, but what is to be found is not helpful, or from what I can tell, even related (other than incidentially having the keyword in the text). There are a hundred thousand hits of some users wanting to use FaceTime or iMessage having trouble activating the service. This is not my issue.
- I do not want to use FaceTime/iMessage, and I do not want to troubleshoot why FaceTime isn't properly activating or whatever. I explicitly deactivated both FaceTime and iMessage along with everything else that can be deactivated and isn't necessary to use the phone as a phone ("speak into the little hole, listen on the other side").
- The phone has been working fine with no disturbances in this configuration since mid/end of April, until suddenly messages started coming in 3-4 days ago for no obvious reason. I did not use FaceTime or iMessage, or attempt to use it, nor register/activate or request anything.
- No longer getting messages now. I didn't do anything to fix it.
Since about three days, I'm getting suspicious SMS sent to my phone (iPhone) in 12 hour intervals (seemingly from a UK number, but it's quite possibly, even probably, spoofed).
There is no "obvious reason" from my side why I should be getting these. The phone is a corporate phone, number is not "publicly listed", the phone is not being used for "anything not business", in particular none of this modern social media share-everything crap (facebook, twitter, and whatever you call it).
Newest firmware, bare minimum of apps installed on the phone (even removed anything Apple installs by default that you can remove, disabled what you cannot remove), every setting is, to the best of my knowledge, set to "paranoia mode". I did not contact anyone (not anyone not-within-corp) recently, nor disclose my number to some internet merchant or similar.
Thus, the only logical explanation seems someone is script-kiddying random phone numbers with some unknown-to-me kind of exploit (but if someone is probing random numbers, why am I getting these over and over again in 12 hour intervals?).
The Messages look like this:
REG-RESP?v=3;r=123456789;n=+491234567890 s=123456789ABCDEFFFFFFFFF01234567890ABC 01234567890...
(I replaced the digits with 12345... in case they're something "sensitive" -- for all I know these messages might as well be some highly illegal unilateral communication accidentially sent to the wrong guy's phone. The obscure 48-digit number might be a code for "terror strike tomorrow" to this scary Maghreb guy living down the road, you never know. So I wouldn't want to publish the actual numbers on the internet.)
The first number is seemingly random, the second number is the phone's number including country code, and the last number is an always-constant prefix of 10 hex digits followed by FFFFFFFF, followed by a seemingly random 48-character hex number.
The messages somewhat look like a query string that you would expect to be sent to a REST service (without host or path). Except my cell phone is, well, a cell phone, not a public web server.
The iPhone doesn't seem to know what to make of the messages either, it simply displays the message text as-is (well, luckily...).
There does not seem to be any suspicious outgoing communication from the phone. Turning off WLAN, the bandwidth quota shown by the provider does not seem to grow. With WLAN turned on and wiretapping on the router, one of the other odd connection to Apple is made every now and then (probably checking updates?), and nothing else.
Is this kind of request aiming at a known exploit in some phones? Seeing how the Apple phone doesn't seem to understand it, maybe Android phones?
Otherwise, any idea what this may be? Botnet control messages?