My partner has been having this issue with his 2nd Sprint Pre for almost a year now.
While he is in a call, the phone places the other person on hold and then tries to call that person again on a conference call. This happens at random and there doesn't seem to be any other things going on. He has doctored the phone and it didn't fix that issue.

Thanks

I see here, there's a similar issue from 2 yrs ago with no solution.