You may also try the Conference Call Dialer, https://forums.androidcentral.com/e?link=https%3A%2F%2Fwww.anrdoezrs.net%2Flinks%2F100048248%2Ftype%2Fdlg%2Fsid%2FUUacUvbUpU3203648%2Fhttps%3A%2F%2Fplay.google.com%2Fstore%2Fapps%2Fdetails%3Fid%3Dcom.abzdev.confcalldialerstd%26hl%3Den&token=20QKNbrm...
I think you need to use one of the conference call dialers available in the google play. I tried some and the one that works for me is the Conference Call Dialer...
There are some applications on Google Play that help with conference calling. I was playing with some, but the one that really does all the work for me (with many calls per day with various pins) is the Conference Call Dialer...