To the question asked - if you actually are having trouble getting in enough calories you can drink them. Protein shake, orange juice, whatever. Drinking calories is quite easy.
To the question not asked IanG pretty much nailed it. When you have a lot of energy reserves (you're obese) the situation is different than if you have only a few lbs to lose. Context matters.
"Starvation mode" for most people means that your body won't lose weight. This is a rediculous notion. The energy needs of the body will be met and your body will not stop burning fat because you have no ingested enough calories.
Rosesandholly I doubt your trainers at the gym are truly knowledgable on this subject. They may be but based on your interpretation of what they are telling you it seems they are clueless. Here are some questions that help establish context.
How are you monitoring your calories? You're sure you're averaging 900 calories a day? What are you eating?
What kind of exercise are you doing in those 30 minutes?
What is your height and weight?
How are you feeling?
"Getting solid information is easier than ever. Getting misinformation is even easier." - Kaplods
Maintaining for two years and I eat whatever I want - just not however much I want. Details here.