Nextgrid, though at the moment it's still a work in progress (building an entire telco core network from scratch by yourself takes a looooong time), but in the meantime you can get the numbers from my carrier partner directly: https://aa.net.uk/telecoms.html

Those are UK mobile numbers, with SIP for voice and webhooks for texts, and all lookup tools I've found recognise their numbers as "Three" (a mobile carrier in the UK) mobile numbers.

I am not actually sure how those guys interconnect to Three, it could even be literally a bunch of GSM modems with real SIMs in it. My long-term plan is to get rid of the middleman and become a carrier myself (get a number range allocated by OFCOM, UK's telco regulator) and offer both SIMs and API-driven numbers (with no way of distinguishing which is which from the outside).