Let's do some rough numbers:
1) According to WTM using 16 1080ti cards is about 2 ETH/month
2) The power consumption for these 16 cards is about 3 kilowatts (under ideal conditions).
3) Cell phone power feed:
a) 12 volt bus requires about a 250 amp feed. So divide 250A by 0.8 for safety
yields an required cable ampacity of about 320A which would be 2 3/0 (AWG) cables.
b) Say 100 mA feed: you are looking at 30 kV input.
Either 3a or 3b requires alien cell phone technology.
Scam possibilities:
i) One of the above comments, where the cell phone + app is just an interface to a cloud mining site.
ii) Some weird ponzi scheme where the "investor" is "paid" back on a monthly basis, but to the investor's surprise the total income ends up less that what was put in.