just because employer did not contribute to your income whilst out on maternity, does not mean that you can phone up sick for a few months. Thhink of your colleagues and the workplace. If everyone did this, no one would have a job and there would be no one working to pay taxes to pay for unemployment benfits.
if you do nto want to return, then resign. if you are ill, get doctors certification and go on disibility benefit.
If you are not up to work because of depression (PNT) then seek immediate help from public health nurse or GP before your get worse.