Hi @james lewis
To exclude the client issue, we firstly recommend you try to clear Skype for Business client cache file by following these tips:
1)Log out Skype for Business;
2)Delete any sign-in information;
3)Navigate to Users > [Your Username] > AppData > Local > Microsoft > Office > 15.0 > Lync
4)Delete the sip_profileName folder
5)Delete all files in Tracing folder. Do not delete the folder itself
If the above suggestion is not work for you. I have some questions to confirm with you:
1)Are there any error messages when you make phone call in Skype for Business?
2)Does anyone have the same issue in your organization?
3)What is your environment? On-premise or online?
If the response is helpful, please click "Accept Answer" and upvote it.
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