Hi PDoyle1,
Understood. The feature you're referring is mentioned in the "Silence a call" section of Answer or decline incoming calls on iPhone.
Press the side button or either volume button.
You can still answer a silenced call until it goes to voicemail.
You mentioned that you're unable to silence a call when you're on an active call. Does the same issue occur when you're not on an active call? This information will help us isolate the issue.
If you find that these buttons aren't responding for other features, If a button or switch isn't working on your iPhone or iPad may help determine whether this is a hardware issue.
We look forward to hearing the results.
Regards.