This is not the answer that OP is looking for. As pointed out by @Seïfane Idouchach Android browsers doesn't support webcal feed.
Workaround:
- Use http:// instead of webcall://
- Setting header to 'Content-Disposition: attachment' to make it force save the file on android devices. (You may also want to set the filename header).
- Now due to the above two steps, android client would download the file.
- Once downloaded, users can click the file to add them to calender app.