Unlike email, which traverses the public Internet, SMS messages are routed through the private infrastructure of mobile carriers, using the SS7 signaling network rather than IP-based routing. SMS is typically carried between Mobile Switching Centers (MSCs) and Short Message Service Centers (SMSCs) within and between carriers, and not over the open Internet. Therefore, sending, receiving, and interception of SMS messages generally requires access to the SS7 network or carrier infrastructure.
Each mobile carrier maintains a gateway to the SS7 network for interoperability with other networks and to support features such as email-to-SMS, MMS, and web-to-SMS services. These gateways often sanitize or control access, limiting the extent to which public systems can initiate or affect SMS routing.
Routing of SMS messages is informed by the Home Location Register (HLR), a central database in GSM and UMTS networks that stores information about subscribers. The HLR tracks the IMSI (International Mobile Subscriber Identity) and associates it with the subscriber’s MSISDN (E.164 phone number). It also records the VLR (Visitor Location Register) pointer and the serving MSC/VLR’s Global Title (SS7 address), enabling the network to locate and deliver messages to the subscriber.
Without access to SS7 (which is possible, but not easily accomplished for people who don't have a legitimate reason to have access, and the people who do get access to intercept messages can do a lot more than just intercept SMS - and the tier 1 carriers in the US get really, really, really fucking uptight when someone tries, so the people who have in the past managed to do so and successfully used it to intercept traffic (as well as other fuckery) usually have done it using infrastructure of badly-secured MVNOs or through compromise of misconfigured Internet-facing systems of telecom and cellular carriers elsewhere in the world.
So, in summary... If you have to ask how it's done, you don't have the capability to do it - because if you can do it, it's pretty likely that you won't do it - but if you do, it's for a whole lot of money so start saving those satoshis... Once the third world catches on, if you don't speak Russian or live in Africa, you're not gonna do it easily.
Disclaimer: I don't wanna hear you soppy vaginas flap your mudflaps about the mythical status of this shit. Feel free to continue to run game on anyone you wish - if you wanna rebut this, you're welcome to do it too, just make sure you lube up first cause when I stand and deliver the reply, it's gonna hurt your little bussy when I jam it in sideways to the hilt.
At ease, ladies!