The most famous of the Chinese Guarantee Syndicates, Haowang Guarantee, is part of the US-sanctioned Huione Pay, "The Largest Illicit Online Marketplace" according to Elliptic and WIRED. Haowang has shifted their business to Tudou Danbao, but their vendors continue to offer SMS Modem Pools and associated hardware and software as part of their Crime-as-a-Service empire.  Here's an ad for one such vendor (with its translation):

@Annie068a operates a channel dedicated to selling SMS Gateway equipment

Annie offers SMS Modem Pools in a variety of sizes

SMS Modem Pools have a variety of configurations.  The most basic has 8 modem ports with slots for one SIM card each. On the opposite end of the scale, is a 64 port modem with capacity for 512 SIM cards. (Many of those found by the USSS seem to be 32-port modems with 256 SIM cards.) When there are more SIM cards than modem ports message sending rotates between SIM cards. 

The concept, as Annie explains, is that you can route messages from anywhere in the world and have them sent from an SMS pool sitting in the United States and being sent from a US-based SIM, thus having a US telephone number displayed in the caller id.

SMS Pools for Fraud and Phishing

An English-speaking Bulk SMS provider, KathyBulkSMS, also is quite blatant about the criminal nature of the messages she suggests.  Her service also has the ability to send using "Short Message Code" caller IDs. She particularly recommends imitating Coinbase if spamming in the US and says that her recent campaign, sending 170,000 such messages via Verizon, AT&T, and T-Mobile, was "very effective."

Kathy gives other examples, such as imitating Binance and National Australia Bank for the Australian market, but her channel has suggestions for many countries, including Netflix and Crypto campaigns for:  Greece Portugal Austria Ireland Japan Slovakia South Korea and Spain.
 

Cheap SMS Modem Pools and Cheaper SIMs

But what about the SIM cards? Don't worry, there are many Facebook groups, and many more Telegram channels that will hook you up. The Telegram user @Zoom557 posts to many Facebook groups using the new criminal-friendly "Anonymous Poster" service. On Telegram he is excited about the new $5 SIM cards offered 

BaronLiu also uses Facebook to push his Telegram SIM card offerings. 

Here are a few of the Facebook groups (all in Chinese) that specialize in SIM card selling. Notice the sizes: 2500 members, 3600 members, 6400 members, and 8700 members. Most of these groups also offer mass account creation and social media spamming services. 

The same vendor shared the photo below.  This isn't USSS in New York.  This is a deployment in Thailand using a SIM pool to provide Thai-WhatsApp numbers to customers around the world. 

Darcula was well-and-truly doxed by the excellent researchers at Mnemonic.io -- Erlend Leiknes and Harrison Sand.  I've spoken to them both and they did a great job tearing apart Darcula's code and mapping out the credit card theft associated with it!