I had a similar thing going on with a 7.0. If a user was in a call, had second call alerting and if a third call was transferred, the waiting callers got connected.
The fix, though not very beautiful, was to make a hunt group for each user and transfer to that.
Moved the customer up to 8.1, but now they are used to the hunt group twitch, so I don't dare changing it back.
Tried to replicate it in lab, but never got the strange "conference" happening, so it left me with the feeling that it was SW/HW related.
Kind regards
Gunnar
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Hippos have bad eyesight, but considering their weight, it’s hardly their problem