I faced exactly the same issue as bagou01 has described..."I am brought back to homescreen while the call is still going on.... The call screen is notre present either in the task manager". I tried the following two things and the issue has been fixed, however I am not sure which one of them actually fixed the problem.
1. I cleared the dialer's data and cache as suggested by gidgiddonihah
2. I switched off the Google Caller ID feature
Hope this helps someone facing the same problem.