If anybody gets a 3CX QR code, they can load that extension on their smartphone. There is no way for you to see who has the extension loaded on what smartphone, and
3CX PBX V15.5 Update 2 - Client QR code configuration and more
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3CX PBX V15.5 Update 2 - Client QR code configuration and more
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3CX PBX V15.5 Update 2 - Client QR code configuration and more
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Provisioning your 3CX account on mobile – Sentrian IT Support Centre
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3CX PBX V15.5 Update 2 - Client QR code configuration and more
Android: Not getting your calls? - Syncd Technologies
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Provisioning your 3CX account on mobile – Sentrian IT Support Centre