I had no sooner fallen asleep, then the phone rang. I had scarcely fallen asleep when the phone rang. As soon as I had fallen asleep, the phone rang. In all three of these sentences, we must use the past participle fallen.

No sooner had the phone rang than i fell asleep. Scarcely had the phone rang when i fell asleep. The phone rang as soon as I fell asleep. I hope that answers your question. If you'd like to practice speaking/writing and grammar, I'd be happy to help you develop a study plan. Feel free to look at my teaching profile and class offerings and send me a message if you think I can help you.