Charlie Stobert @Quora says it as well as I can.
It changes frequently.
The UE (User Equipment) or "mobile phone" is assigned an address by a process similar to DHCP by the mobile operator in question.
Every time the UE is restarted, or even when moving across an invisible network boundary that you're not even aware of, your IP address is likely to change.
Also most operators would have to use NAT to translate a private address assigned to the UE to a public address in order to conserve public IP addresses and that is completely dynamic. See my other answers on this topic for more information.
I verified this by checking my IP on the device at home. Then at a work location on the same day. They are two vastly different IP's with the same major carrier and I was not in a roaming mode. The locations were under 40 miles apart.
Did you factor in whether or not you were connected to a wireless network (vs. straight cell phone connection). Do you know of any software that might record external IP Addresses? I have a bunch of questions. What's your level of interest in this? High? Middling? Barely?
Oh, and thanks. I've been wondering this for weeks.
Is the IP Address IPv4, IPv6, or both? I'm roughly familiar with v4, but I think it's time to educate myself on v6 since that seems to be the way of things.
One of my "areas of interest", or reason for being interested has to do with privacy. Couple weeks ago, someone sent me a link showing that some online "entity" is collecting the IP addresses of P2P filesharing torrents and posting them online, so that if someone knows "your" IP address, they can run a google search of it, and discover not only that you've been P2P filesharing, but WHAT files you've been sharing. So the idea that "your" IP address has become a means of identifying you online is a privacy concern. So I ran a search of "my" IP address and found a bunch of edits that I made on Wikipedia.
What if I don't want (for example) the people that run Bleeping Computer (or Craigslist, or Disqus, or this forum, or that forum) to know what edits I've made to Wikipedia, and/or what comments I've made in it's "Discussion Pages". What if (for example) you were a secret, closeted Scooby Doo fan, and you put hours and hours of effort on the Scooby Doo Wikipedia article, Scoobying and Dooing to your heart's content, letting your truest and most passionate super-sleuthing crimesolving meddling teenager edit to your hearts content, and then have someone in another area of your computing "life" discover this shameful fact of your identity.
In short, I don't like IP addresses being "permanent". I'd like some idea of how often they change. Yesterday I was in the waiting room of a doctor's office, and I connected to the free wireless to kill some time. No my real identity, my medical records, my IP address and what site's I visited are all together in a common "node" of an unsecured public network. It's a really big deal. Just ask Hillary.
I make this little speech because I asked the same question on another forum and the thread got closed because the Moderator there thought I was trying to do something nefarious, like if they gave me Ye Olde Ancient Secrete of the esoteric IP Address Death Magic, somehow I would resurrect the Pagan Gods of Mordock and Greyhammer, and usher in the Age of Holy Sanctification.
I just wanna know. Does it change every day? Every time I restart the device? Etc... I assume it depends on everything. Manufacturer of the device, service provider, etc... I know I'm not the only person wondering about this, and I wonder if, in response to all those invisible hoards of people that also want to know, some programmer hasn't made an App that "pings" every "x" minutes and makes a note every time your device's external IP Address changes. Thanks for the link to Quora. I'll go read that now.