Two reasons why this is a bad idea:

1) Your phone carrier knows both your number and your first name.

2) Assume that the adversary doesn't know names or phone numbers. How many distinct phone numbers are there in the world, like 10^14, tops? How many distinct first names? Millions, although most of the population is probably covered by 100k first names. But let's say that you have on the order of 10^21 hashes to compute, that's 1000 exahashes. The bitcoin network does that every two seconds.
 

 1. People who care about their ISP knowing their npub won’t use this.

2. You can’t compare sha2 hashing to what im talking about here, they are not the same thing 

 1. Sure, but those who don't care can easily get identified, and therefore the people that they interact with/follow are much easier to identify by deduction.

2. What's the material difference?