Basic255 the website is https://silent.link
They give you an eSIM of a foreign country and you connect this to any network available to you (they even habe a price list per country and provider and two different tiers of data SIMs so you can get what's cheapest for you). The "anonymity" is limited to
- the SIM being in roaming mode and therefore harder to identify for the network provider since they get fewer information from it
- the company you buy the eSIM from doesn't know who you are since you don't give them any KYC information
SIM cards and especially eSIMs are still inherently bad for anonymity, here are a few examples why:
- they get access to your real hardware identifiers and there's no way around this
- currently for eSIMs to install, you have to give Google privileged access as well
- the mobile network is built as a tracking network (it needs to track your phone in order to know where to send the data)
- triangulation will always be possible with current tech as soon as you are out of airplane mode
- if you make one mistake, your anonymity is gone. Like leaving your phone out of airplane mode for one night, or even putting it out of airplane mode too close to your home, or sending unencrypted identifiable data over the network (like you would do by using Telegram for example).
So it's hard, really hard to have mobile data and stay somewhat anonymous. A mobile router that allows you to change the IMEI (illegal is some countries) can mitigate it maybe, but triangulation and decryption will be your enemy at all times.
In the end you need to know what you really want and define your threat model. There are levels of privacy and anonymity that are very inconvenient to achieve and probably not worth it if you don't happen to be on an Edward Snowden threat model. For me using a silent.link data eSIM on a GOS phone is good enough.
Edit: Typos