Ibm — Offer letter not released after Background mail Verification

Website:

IBM.com

Given 2 rounds of Interviews with IBM. The second round on the 4th FEB received the call from HR that I have been selected so I have given my resignation based on the call and confirmation. Received the Snachapp verification mail I verified that day only. Tried to contact HR several times contacted several times mailed several times still the offer letter has not been released. This is very much unprofessional from IBM