Hi Corina1 welcome to the GoTo Community. If your attendees only dial into the conference call there's no way to identify them and include them in the attendee report.
What you can do is remove the dial-in audio option because the only audio will be the built-in 'computer audio (VoiP).' You can do this in your general GoTo Webinar Settings under Audio, or under the specific Webinar under Settings and Audio.
Alternatively, you could also make it clear on the registration page or the confirmation and reminder emails that attendees must join from a desktop or using the mobile app otherwise their attendance won't be recorded and they won't get a certificate.
