iirc, the calling number is reported from the calling device, not the service provider.
I remember playing around with that on my rooted Android phone years ago (around Android 2.0-2.2) in germany.
It would probably be possible to discard this information from the client and overwrite it as the service provider, but they weren't doing that at least back then. It would also be costly (like a MitM proxy overwriting headers)
That was around 10yrs ago though. Might have changed by now.
It's not costly. There's a lot of similar processing already happening, the proxies you mention are not mitm - they're part of the system. Most providers do ignore what you send them about your external caller id - in many cases you don't even know the correct one. Blind forwarding the cid is a bug rather than something people decided to do for cost or other reasons.
I remember playing around with that on my rooted Android phone years ago (around Android 2.0-2.2) in germany.
It would probably be possible to discard this information from the client and overwrite it as the service provider, but they weren't doing that at least back then. It would also be costly (like a MitM proxy overwriting headers)
That was around 10yrs ago though. Might have changed by now.