Her anxiety is over the fate of a young, well-built man waiting in her ward. Originally detained under the Mental Health Act as a risk to himself or others, he absconded from the hospital shortly after Christmas.
The police have just found him miles away in south London. He is so disturbed that no one doubts he needs now to be on a locked ward. But the hospital's own five-bedded locked ward is full. Dr Gupta keeps punching the buttons.
Suddenly there is an explosion. The man is off, out of the ward door, bounding down the stairs to an access door which locks magnetically to keep unwanted visitors out. He simply bursts through it, busting the hinges, pursued by two of the ward's four nurses, neither of whom is a match for him physically.
A chase ensues. Dr Gupta keeps phoning. Five minutes later, downcast, the nurses return. "We cornered him in a corridor," one says, "but he just came at us swinging and was away."
The next phone call will be to the police. There is nothing more the hospital can do except tell its community teams that the patient is again on the streets. No one can be sure quite how much of a risk he is to himself or others. He was not in long enough to be assessed.
Dr Gupta, 27, is at the nurses' station on Nelson ward in Lambeth's South Western psychiatric hospital. The tension in her voice is palpable. So is the atmosphere on the ward, taut like a violin string about to snap.
A woman is weeping, uncontrollably, great rivers of tears pouring down her cheeks. A man laughs, hyena-like, when anyone looks at him.
There are 19 beds on this ward. But it is currently responsible for 27 patients. The others are out "on leave" - trial periods of a day or two back in the community. They may do well. Or they may break down. It is better they do well. Their beds are filled. And there is a waiting list of six patients whom the community psychiatric teams would like admitted.
This is a service that across Britain's inner cities is "in crisis", Dr David Roy, medical director of the NHS trust in Lambeth, says - a verdict the Mental Health Act Commission first delivered two years ago and repeated last month, warning that if anything, things have got worse.
The court diversion scheme, aimed at getting mentally ill offenders into hospital not prison, has filled the secure beds to the point where Dr Roy's service now has 25 local patients in private secure hospitals around England, Scotland and Wales, at a cost to the unit pounds 80,000 per place per year. Meanwhile, mental illness in the high unemployment, socially deprived inner cities has continued to rise.
Consultant psychiatrists are leaving the inner city because of stress. Dr Roy's service has lost two in the past two years and been able to attract only one - a locum, who will move on.
Dr Roy's solution is not more acute beds, but action to take pressure off the top and bottom of the system. More secure beds to handle the mentally disordered offenders and provide immediate access to those needing medium secure care. More 24-hour staff homes, respite and emergency services out in the community which cost less than an acute hospital place and are better for patients.
At South Western itself, some help is on hand. Next month a high-quality pounds 20m development opens, with seven extra locked beds. But with 25 patients out in the private sector, it does not take a genius to work out that Dr Roy's problems will be eased, not solved.