The idea of a phone line having a specific number is a consumer thing. In the business world your connection is some number of concurrent channels. The business

The idea of a phone line having a specific number is a consumer thing. In the business world your connection is some number of concurrent channels. The business’s PBX decides what number should be displayed as caller ID when setting up an outgoing call on one of those channels. Inbound and outbound routing may be different (just like a domain’s outbound SMTP server may not be the same as its MX record) so allowing only the numbers that are routed to that PBX doesn’t work. You need something like the SPF record. That thing is called STIR/SHAKEN and it’s only being rolled out now.