In the Skype for Business main window, click the arrow next to the Options button, and select Tools Audio Device Settings. Under Audio device, choose the device you want. Click the green arrow next to Speaker to hear a sample tone, and drag the slider if you need to adjust the volume. Scroll down until you see Screen Recording and click the Skype checkbox (Skype will close to apply the changes). Reopen the app and make a test call to see if the problem has been resolved Unfortunately, it isn’t clear if the issue was because of Microsoft’s new update as many apps are behaving weirdly on Apple’s new macOS Catalina.
Skype for Business uses these ports and protocols:5061 TCP (SIP) - Used for SIP/MTLS authentication of A/V users. Communications flow outbound through the internal firewall
443 TCP (HTTP) - Used by Skype for Business clients/Lync connecting from outside the intranet for SIP communications
1024-65535 UDP/TCP - Port range used for inbound and outbound media transfer through the external firewall
6891-6901 TCPPort range used by Skype for Business/Lync clients for file transfer
Make sure your firewall is not blocking these and it should work. I suspect that it is a firewall issue since it works internally on the same network.
Summary: How to install and test the Skype for Business for iOS app.
The Skype for Business for iOS app brings Skype for Business presence, instant messaging (IM), and voice and video calling to iOS devices. Users with Lync 2013 will either get the updated app automatically or be prompted to update it manually, depending on their user settings. New users can download it from the Apple App Store. The Skype for Business for iOS app is only available on iOS versions 8.0 and later.
Installing Skype for Business for iOS
From your iOS device, tap App Store, and search for Skype for Business for iOS.
Tap Download > Open.
Sign in to the app.
Verifying mobile client installation
After you sign in successfully, use the following tests to verify that your Skype for Business installation is working correctly on your iOS device.
Search for a contact in the corporate directory
Skype For Business Mac Test Audio Driver
In the Contacts list, tap inside the search bar at the top, and begin typing the name of a contact that exists only in the global address list (GAL).
Verify that the contact name appears in the search results.
Skype For Business Mac Test Audio Recorder
Test instant messaging and presence
In the Contacts list, tap a contact.
Verify that an instant messaging (IM) window appears and that you can type and send an IM.
Test dial-out conferencing
In Outlook, schedule a Skype for Business meeting.
On the mobile device, open the meeting invitation.
Click the Join Meeting button.
Answer the call from the conference service and verify that you are connected to the meeting audio.