This from Apple Toolbox:
“Unfortunately, at this time, there is no way to separate the volume of your iPhone’s ringer for phone calls from the alert volume for things like alarms, texts, emails, messages, and other alerts–their sound is set at the same volume.
And we really don’t know why Apple ties both the ringer and alerts volume to the same volume slider. It’s doesn’t make sense for how we use our devices today!
So you cannot lower the volume for any alarm you use to wake up in the morning (or for whatever) without lowering the volume for incoming calls.
The current recommendation is to turn down the ringer volume slider each night, then turn it back up in the morning so you don’t miss any phone calls. Yeah…not ideal!”