Did you record it?
You planned it properly, asked relevant questions, and overall had a good interview session, but did you record it? In a telephone interview especially, there is a high chance that certain important verbal or non-verbal elements are left unanalyzed. This could be down to several factors. Did any of the following keep you from making an accurate record:
Uncontrollable distractions, such as a call from your manager
A temporary and slight drift in your attention during the interview
Too long a discussion
Disturbance in the telephone connection
Noisy background at the candidate's end
Network issues at the candidate's end
If any of this happened, and the interviewer was not recording the call in some form or another, it is highly probable that the assessment of the interview session or the candidate may be flawed, resulting in great wastage of effort at both ends.
One of the worst ways of recording the notes of a telephone interview session is by scribbling on the resume itself. Scribbles...