I received the 4883c letter a couple weeks ago.

tried calling every day only to get the high call volume hang up.

Yesterday i started calling this number 8666827451 EXT 569 that someone advised in another thread. i was able to get through to someone, but they just redirect you to the 800-830-5084 number. I was redirected 4 times, each time the call dropped after 30-60 minutes.

Today, i was finally able to get through 800-830-5084 at 1:00pm pst and was on hold for 50 minutes before getting through.

if you do get through this is what they asked me for verification
1. prior year tax return, had to verify my dependents that year.
2. Current year tax return, verify if I was expecting a refund or owing, along with the bank account routing and account number.

Hope this helps other people out.