Delete all spam or missed calls at once on iPhone

No, that is not a feature. You have to select the calls you want to delete. This is likely to avoid accidentally deleting all calls, when there may be some you didn't want to delete.

You can provide feedback to Apple if this is a feature you'd like to see, but I wouldn't hold my breath on this one as this forum would be flooded with folks complaining they accidentally deleted all and now they need some back --> Feedback - iPhone - Apple

You need not delete one at a time.

1. Select the Unified View
2. Select Missed Call
3. Edit
4. Select and drag all the way till the end
5. Tap Delete

See the pic below for Missed Call deletion all at once. (Do the same Process for unknown callers too)

Thank you, SravanKrA! That worked perfectly!