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.
Edit: I meant moving off cloud to Hetzner, Vultr, DigitalOcean.
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.
The loaded costs should have the numbers run.
It would be a fascination under the covers look with signal.
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.
Also, FOSS and managed aren't mutually exclusive.