Both of the phrases you provided are polite and appropriate responses to explain why you couldn't pick up the phone when the person called. Either one is fine to use, but you might consider some minor adjustments to make the response sound even more natural:
- "Sorry, I couldn't speak at that moment."
- "Sorry, I couldn't talk right then."
Both options convey the same message, and it's up to your personal preference which one you choose. The most important thing is to acknowledge the missed call and offer a polite explanation. If you wish, you can also add a brief reason, like "I was in a meeting" or "I was driving," depending on the situation, but it's not necessary unless the person specifically asks.