> we can rent server infrastructure from a variety of providers like Amazon AWS, Google Compute Engine, Microsoft Azure
Moving off cloud services to lower-cost provider like Hetzner, Vultr and DigitalOcean might provide a lot of cost savings.
I also imagine they're using managed SMS services from one of these clouds, and moving off them to a combination of local SMS gateways in each country can also further reduce costs (and in one case I've personally observed, by upto two orders of magnitude). This obviously pushes a lot of complexity on Signal's side, but is usually worth it.
In business, you get what you pay for. Cheaper hosting might raise more issues that need to by handled by your employees, who also are expensive, and also the organization's focus gets disrupted. The hosting company / cloud vendor has an enormous economic advantage, with access to the entire hardware and software stack, the engineers who built it, people whose full-time job is operating it. Often it's cheaper to pay more for better.
As I have to explain about open source, 'Free is only free if your time is worth nothing.' (And I use a lot of FOSS, it just not always the solution.)
This is the worst take in technology. The main value of FOSS is freedom, not time or money savings. For many people freedom is more valuable than either.
As I understand it, you have to often use multiple gateways based on which one is cheaper and can deliver your message to the recipient, and also take care of handling retries in case one gateway fails. This is not something you typically want to handle if you're not aware of it, and the process of having to talk to each vendor and figure out their limitations is tedious.
There's a lower bound on what these services can charge in the form of interconenction fees charged by the mobile service providers delivering the messages.
In the US, that's effectively zero due to the US phone infrastructure largely using a shared-cost model, but in most other countries which use "sender pays", these fees can be significant.
Moving off cloud services to lower-cost provider like Hetzner, Vultr and DigitalOcean might provide a lot of cost savings.
I also imagine they're using managed SMS services from one of these clouds, and moving off them to a combination of local SMS gateways in each country can also further reduce costs (and in one case I've personally observed, by upto two orders of magnitude). This obviously pushes a lot of complexity on Signal's side, but is usually worth it.