The virtual numbers come from a few different suppliers, one being Twilio for example.

The physical SIM numbers are infrastructure that we built, using racks of GSM modems, SIM card switching devices, and our own software to manage everything. The SIM card switching devices reduce maintenance requirements since when a customer no longer needs a SIM, we can electronically switch the modem to a new SIM without needing to visit the datacentre. We just have to go every so often to replace the used SIMs before the pool runs low.

Wouldn't number verification systems (for example, Facebook) eventually start blacklisting your numbers? I have seen this happen with VOIP numbers. Do you have a plan to tackle this?

The physical SIM numbers are SIMs from major UK providers, and are never re-used (for clarity: if you buy a SIM number, you get a SIM that has never been used by anyone else, and it will never be used by anyone else after you).

From the perspective of a number verification system (or anything else sending an SMS to them), they are completely indistinguishable from a mobile phone and prepaid SIM, since that is what they are.

The virtual numbers, while also never re-used, are often blacklisted by services that do SMS verification due to coming from a range designated as "VoIP" or "virtual", and we explicitly disrecommend their use for verification, although some people do use them with varying amounts of success since they are cheaper than the SIM numbers.

> it will never be used by anyone else after you

This isn't true; UK mobile numbers are re-used after a period of inactivity by the operator themselves.

Unless, of course, you're keeping the number/line live in permanence? But I can't see how your service would make that work financially.

If you re-read what I wrote, or perhaps quote the whole thing, you'll see that I was referring to the SIM, not to the phone number.

Of course unless phone numbers perpetually get longer, they have to eventually get re-used. We don't implement any re-use but the operator will indeed eventually recycle them.

Great to know that the SIM numbers are not re-used across customers. Would this also mean that I have no way to re-claim a previously owned number?

You can keep a phone number as long as you like. If you accidentally delete it or allow it to expire, then we can probably, on a best-effort basis, recover the phone number for you if you alert us quickly after you make the mistake. Once we have done a resupply of the SIM card pool the SIMs that have been used and are no longer in use are destroyed, at which point we can't get the number back for you.