SfB - some callees cannot hear me

Weird situation that I need help troubleshooting - Using Skype for Business client and VVX desk phone,  I make a call to a particular business and they cannot hear me.  I call that business from my cell phone and they can hear me.   Using Skype for Business client and VVX phone, call a different business number, I can be heard.  This one business (and a couple others, including a fedex office) cannot hear my voice.

How can I find out if it is MY skype for business issue or if it the business's phone issue?