First off, theres no need to worry about any RINGR settings for the audio format etc. as you can create any versions you need after the conversation.
Next, look to your and your guest's recording setups. The microphone and recording environment are the biggest factors rather than whether you/they use RINGR mobile or RINGR desktop to record. Its worth asking your participant to have a quick think about where they can record, preferably in a quiet space with soft furnishings or curtains to avoid any reverberation.
If you participant is joining on a computer or tablet, its best to try and avoid a speakerphone setup. We use echo cancellation in RINGR to ensure that you can record on a speakerphone or mobile handset (echo cancellation is also used when you make a normal phone call), but its best to try an avoid it happening altogether by making sure your participant doesn't have the volume set too loud or uses headphones during the call. You can always give them a quick sound check at the beginning of call in the 'green room' before you hit start recording.
You can further improve sound quality by using a higher quality or better placed microphone, such as a headset with a carefully adjusted boom mic, or the Apple earbud style headphones with an integrated mic that come with most smartphones. RINGR desktop will support any mic/headset you can connect to your computer, while RINGR desktop supports TRRS mics (those that plug into the headphone socket on your mobile) or those connected via the lightning port on iOS devices. We often recommend mics from IK Multimedia, such as the iRig Lavalier mic (which you can connect your headphones to) or the iRig Pre, which will allow you to connect a studio condenser mic with XLR connector and will supply phantom power.