1. Use your computer's Internet browser and go to http://webcal.fi.
  2. Select Calendars > Manual subscription and tap on Show URL next to the Week numbers on the list. 
  3. The calendar URL appears at the top of the window in red. Select the address and copy it (Edit > Copy).
  4. Sign in with your Microsoft account ID to calendar.live.com. This is the same ID you are using on your phone.
  5. When you see your calendar, tap Import from the top and select Subscribe. Paste the address (http://...) into the Calendar URL field and give your calendar a name.
  6. Subscribe to a calendar.
  7. When your phone syncs with Microsoft account next time, the option for the week numbers will appear (Calendar > Settings).

To remove the calendar:

  1. Sign in with your Microsoft account ID to calendar.live.com.
  2. Tap Tools from the top and select Options.
  3. Select the calendar and tap Delete.