That would be your landlord.
You will have to give written notice with permission to enter to repair the work. Make sure you receive a stamp or dates signature of receipt of the written notice and keep a copy of it for yourself.
After which your landlord has 10 days to fix the issue or find out the extent of the repairs and return to you, in writing, when the repair will happen if needing to be done by an outside source. In this case, the plumber.
If the plumber is coming before the 10 days, no extra notice is needed. If coming after the 10 days from receipt of your letter, they will have to give you 24 hrs written notice of entry for the plumber.
If after 3 attempts, you have not gotten the work completed, you can make a T-6 application (double check for your area) from the Landlord Tenant Board to have the landlord ordered to fix it. In some case, you may pay for the repair yourself and then add that amount in the application suit against the landlord. I would of course double check this for your area and with a lawyer before paying yourself.
I'm going to assume you are a market renter and not in any type of social housing as those come with other options as well.