There are several ways by which two-factor authentication (2FA) is implemented. The most popular are sending a SMS message or e-mail to a pre-determined and approved mobile number/ e-mail address, or using a physical (sometimes software or app-based) cryptographic token such as an RSA SecurID or Thales dongle that rotates the derivate six-digit key,
Some financial institutions do support sending 2FA user-side factors by e-mail, by inbound voice call, or by outbound call to a live agent that will verifiy the caller and permit the action to continue. Wells Fargo is a well-known example of a bank that still uses e-mail transmission as a 2FA mechanism, but increasingly they have changed the operations threshold at which it will become mandatory to execute 2FA validation. In my experience, it seems that their OLB web application acts similar to that of American Express, which continually examines page clicks and zone movement within the web app, and assigns what I can only assume is a cumulative risk score for sequences of action which will trigger mandatory 2FA in degrees up to and including forcing a sign-off if cancelled, and returning an error if sign-on attempts are made during a cooling period of 20 minutes to one hour of more.
So, knowing that, and knowing that e-mail is inherently insecure, that leaves pretty much every bank on the planet to use SMS-delivered credentials, and contrary to what ANYONE will tell you otherwise, there is only one pathway by which SMS 2FA can be manipulated or intercepted, and even that single path is out of reach for most anyone that would indeed, claim to believe otherwise. Why? Because SMS messages are very, very special. When your mobile handset has an SMS message to send, it dispatches it to the telephone network system back down the handset's two-way RF link to the tower, and from there the message is examined and formatted correctly for the destination MDN, to which it will travel not the Internet, not the public switched telephone network part that handles your phone calls, down a separate circuit link over the telecommunications infratstructure by means of Signaling System 7, or SS7 links, that are not accessible by end users of the telecom networks, nor by people who just use the data connectivity of said networks.
That being said, you can't just drop onto a SS7 trunk and sniff away. You have to have the infrastructure or access to such equipment, and actually have access from a telecom carrier and an agreement to intercornnect to their property. That puts a really, really, really high bar to entry for most anyone who wants to even attempt to do it, as even if all the above factors are satisfied... Without at least a mildly technical understanding of how and what and why, It's still super fucking complex to understand the differences between an ISUP, STP, and MTP, point and sub-point codes and point code routing, how home network association and off-network roaming requests work and how to address those things via your configuration.. ALL OF THAT, before you even can think about IMEIs, ICCIDs, and the fact that all of the above has the end result of one of two outcomes: fail miserably and have to answer a lot of very incriminating questions as to your motive and goals, or two, success and stardom among your peers as a bad-ass motherfucker, until someone in the fraud control department of the carrier you are attached to starts noticing the unusual inward roaming of handsets that almost immediately go outward back to their home network without placing any calls. Then you'll have to answer to not just that telco, but probably may end up involving law enforcement if the end target figures out something funky happened when your interconnect provider telco notifies the wireless carrier that the target originated on, who will definitely notify the end user, et cetera, et cetera, ad nauseum ad incarceretum.
SO WHAT DO WE DO NOW? Simple. White-label MVNO cellular carriers exist, that will (for a fee, as part of their services you are white-labeling) permit and provision your needed SS7 interconnections for things like providing "on-network" data services like e-mail, mobile data proxies/MMS proxy, et cetera. One of the more notable or "famous" ones will provision single SS7 MTP links solely for SMS transmission/SMS blasts, and the manner in which they provision your new MTP trunk to your "switch" is quite perfect for the job if you are willing to work a little to build out the appropriate facade. No pain, no gain, you pussies.
There is another way that requires bigger balls and more bravado, and that's to do your homework on your target, and execute a social engineering attack over the phone, or if you're got huge-ass goat nuts and a platinum tongue, you can do it in person with the carrier, and get them to swap the IMEI of your device for the target one. This has the advantages of human-based (and thus defeatable) defenses, or sometimes additionally some information like last 4 SSN, MMN, security words, et cetera may be needed if you are doing it by phone, but in person you just need a good solid clone of an ID and knowledge of the target. Personally, having experience with SS7 and telecom in general for years professionally, I have the skillset and can prop up a damn good facade that would make the SS7 interception route a home run hit out of the park for one or two high-value targets - it's a shit-fuck-ton of work, and you'd end up burning some useful resources along the way but for the right target and ops goals, eh. Maybe.
You other toads and princesses, however.... The ones among you that trap-flap about things you are ignorant of can continue to bullshit your way through conversations about this stuff and claim some kind of scammy software or method or what the fuck ever can bypass this intentionally dead-end-wall-making tech that by design gives all the drooling idiots positive-bellyfeel about the masters of their hard-earned slavewages so they can sleep at night and make more money for the assholes in the high rises in lower Manhattan. If you took the five seconds to understand the mystery behind the tech, you'd know I am right.
In conclusion, if you want to ride high and go for the top rung on the ladder, it pays well to do the easy route, but if you want to get to the top so you can get the fuck down off of my obstacle Private Pyle, you're gonna be doing the long march at dawn till you puke, but at least when you show up at the end, there's tons of Eskimo pussy that's nice and cold, full of hard fucking cash.
Goodnight, ladies.