Three minutes of Sheila just saying 'yes/no' in a two way phone conversation is a long time.
If she was a shy 6 year old maybe. But she was 27.
It seems the multiple strong side effects of Haloperiodal were really effecting her at this time. As suggested by Wilkes, CAL, Bamber, PB, Julie, the COA, Michael Horsnell & the crime scene.
Even Sheila herself had requested her dosage was reduced.
Bamber's biggest problem he would envisage would be waking her up.