Payment Plan for Self Assessment

This is my first self assessment and I am trying to set up a payment plan. I have tried online but it says I need to submit my return - which I already have? I have also tried phoning but am constantly being cut off before I've even spoken to anyone. I have made a lump sum payment to try and pay a portion of it but not even sure that I have done that properly. Very anxious and stressed. Does anyone know how a payment plan can be set up/what happens if no-one answers the phone?

Hi AliPurto

It can take 72 hours for our systems to update once you have submitted your tax  return.

If you are still unable to set the plan up on online you will need to contact our helpline again.

If you cannot pay your tax bill on time

Thank you.