PSTN interruptions on iOS

When using the iPhone with the latest client installed, and on either an ad-hoc call, peer to peer, or scheduled meeting, when a call comes in over the PSTN to the cellular number, this knocks out the Skype for Business call, or at best puts it on hold.  This is for me at worst a minor annoyance, unless I am speaking, but for the rest of my user base, this is a major issue.  How do we stop this from occuring?  Are we missing a setting on the iPhone that auto-ignores incoming calls for the cell phone whislt using the S4B client for a VoiP call?