I am sure there are many other solutions that are similar and more cost effective than 10k or even 2k. I would imagine that these solutions would have a secure web interface that allows approved users to assign telephone numbers, both fixed and mobile, to flat numbers.
The available technology in this area has experienced a paradigm shift. You only have to look at concepts like https://en-uk.ring.com/products/video-doorbell-2?gclid=EAIaIQobChMIltC92Iv93gIV1uR3Ch3B4gyLEAAYASAAEgIcT_D_BwE to realise that there might be commercial solutions that use a similar principle perhaps. Am happy to look into it a bit further.