I would suspect the end user is making a call, not completing it, and then not completely hanging up, and making another call.
At the point where they do not completely hang up, they are creating a conference call.
Thus, when they hang up from the completed call, the system rings them back for the first call.
I am quite sure that is their problem.
The evidence of this is that when they make the 2nd call, they will hear STUTTER DIAL TONE.
I would caution them to listen for the STUTTER DIAL TONE, and if they hear it, HANG UP FOR 20 or 30 seconds before making any othing calls.
I have seen this MANY TIMES in the past.