VOIP vs Dial In - Audio Quality Issue

VOIP vs Dial In - Audio Quality Issue

We did a test as we are new to Zoho....

The presenter had a problem getting in with VOIP, the attendee did not.
The presenter then dialed in and both parties could hear each other, but the quality was severely lacking.
The presenter was using external mic and attendee using built-in laptop mic.

What could be cause and possible correction to the poor audio quality?

Suggestions needed right away or we'll have to find another provider for this afternoon.

Thanks, U