dtmf-io

Hardly major, but our[0] entire stack except the frontend is Rust.

[0] https://dtmf.io/

The app actually supports iOS already (it's built with Flutter) but we changed legal entity and made a mistake with the app transfer process, so it's temporarily unavailable. We're hoping to have it back on the store in the next week or so.

The problem is that free public WiFi is wildly variable in quality, whereas 4G is usually quite good. Most of the time I just tether even if there's WiFi available since I can use my data plan in most countries and the WiFi is usually disappointing.

Also, many countries now require public hotspots to verify the user by getting them to receive an SMS. In some countries it even has to be a local number, because they have compulsory SIM registration, so requiring a local number lets them tie the Internet usage to an identity.

The UK notably doesn't, and most providers are still doing "roam like at home" with the EU despite the UK having left.

Most likely they have heard of the MVNO/"service reseller" LycaMobile, but how does that respond to their comment at all? LycaMobile still require you to register your ID with them in countries that have that rule.

Sorry! We're always adding new number types and countries though, so keep an eye.

You rent a virtual mobile, landline or toll-free phone number, or a physical SIM, via the website or Android app. The phone number is then yours as long as you keep paying the rental fee. (After you delete it or stop paying, we don't give it to anyone else - we return the number to the upstream phone networks who will, as with all phone numbers, 'quarantine' it for many months and then eventually re-use it.)

You can then send and receive SMS, make and receive calls and send faxes using the numbers via the website or the Android app.

Pro accounts can also make and receive calls via SIP.

We only verify them for completeness; then we submit them to the relevant regulator and wait for them to give the documents the OK. We don't assign the phone number until we get the OK from the regulator.

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.

It requires infrastructure, so it's not trivial, but we're investigating some options. Obviously it would only be practical in other countries that don't require SIM registration, or if they do, it needs to be possible to do it via some kind of API or other reasonably automated means so that we can build a system to let people register their SIMs.

In the UK, a SIM card can be purchased without registration. We simply allow you to do that online and give you remote access to your phone (modem). One SIM card and one modem is used by one customer. It isn't really any different to walking into a shop and buying a phone and a SIM card.

We comply with these regulations. To get a German landline number via our service, you must provide proof of an address in the same locality, as well as proof of ID. We supply this to the regulator, and once they approve it, we provide the number.

Many countries do not have these requirements, and for these countries you can just "click and buy".

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.

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.

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.

Thanks - we'll add this information.

The per-minute charges are fixed for a given number type for inbound calls. They're cheapest for landline numbers (e.g. just under EUR 0.01 per minute for a UK landline), very slightly more expensive for mobile numbers, and quite a lot more expensive for toll-free numbers (since you're paying the cost for the caller). For outbound calls, it depends where you're calling. There are volume discounts available.

We guarantee one simultaneous channel per phone number for Basic accounts. For Pro accounts ($10/month) we guarantee an additional 8 simultaneous channels which can be used across any of the phone numbers as demand requires. Those are guarantees and our system will often allow bursting above them if capacity allows.

For Business accounts (negotiated minimum spend) we can negotiate any number of simultaneous channels.

Pro & Business accounts have SIP support, both registration and directing inbound calls to a SIP URI.