Did this invitation for a telephone "fit for work" assessment come from your employer? If so, and as you haven't yet claimed any benefits, I imagine it is your employer trying to establish if you are fit for work in some capacity or other, maybe a different job within the same company, or part time, or with extra help. If you are deemed unfit for work they may terminate your contract. If you are deemed fit for some work, they may offer you something part time or less stressful. It will be up to you to decide which avenue you wish to go down - never returning or returning part time/less stressful or whatever. You need to think about it carefully.