That's insane! I can't give a definite answer but thinking aloud, I suspect they have a software-based phone switch (Asterisk, 3CX, FreeSwitch etc.) that's wrongly configured to present the called number as its CLIP, and the SIP gateway service they use doesn't check for valid CLIP in the SIP headers. I doubt that it's the result of a hack but yes, they need someone competent to give it a once-over.
I can't speak for anywhere else but crazily in the UK CLIP is done on an honour and best-effort basis - the network is only obliged to request that the subscriber sends correct information, they don't have to enforce it.