Well, using a reversed nibble format allows you to have a different nameserver at each level distributing the load and ownership. If as a telco, I have the block of phone numbers +14155551000-+141555519999, I can easily have my nameserver delegated for it as 1.5.5.5.5.1.4.1.e164.arpa.

IPv6 and IPv4 reverse lookups work the same way.

Besides, it isn't like users are typing this in directly. Might as well get in all the flexibility you can. You could do things like add extensions and have it route properly without any extra work or have a country with both 7 and 8 digit phone numbers.