I'd write them one single email called Situation Report. Detail in full exactly what happened to you, what the doctor instructed you to do, what medication / resting etc you are having.
Be very clear about your current pain levels, nausea, difficulty walking, getting to the toilet, difficulty in being evacuated in the case of an emergency, difficulty thinking and concentrating, anxiety, anything else you're enduring.
Tell them 'I will let you know nearer the end of the current signed off sick leave period how I am and what the doctor tells me to do.
Meanwhile, this is the situation: I'm going through a healing process that requires rest and peace and quiet. Trying to do too much too soon could set back my healing and lead to a longer period signed off.
I can't tell you when that will be, but I guarantee I will keep you informed of any development or change'.
Then when they call, don't answer. Email only - that way you have a clear record of every time they try it. If they left a voice mail, type it out in your answer. Keep the records.
Reply with 'I refer you to my email dated xx. The situation remains the same. As I say, neither I nor the doctor can give a definite statement about when I can return. Any changes, you will be first to know.'