I think the idea was more than just for display purposes. There was an idea that the telephone-switching address space and public Internet could be combined, b

I think the idea was more than just for display purposes. There was an idea that the telephone-switching address space and public Internet could be combined, because the devices were converging towards each other. So an address in .tel would be a phone number, and also all active telephone numbers in the ISO numbering scheme would be valid addresses under .tel. You'd be able to put in DNS records to forward "friendly" names to phone numbers, and also set SIP endpoints for PSTN numbers. It was never really clear who was going to run all this, i.e. would it be the phone company, or some outfit like Network Solutions / Verisign... and I think that's part of why it never really happened. And phone numbers and DNS remain separate address spaces.