Can you please check whether you can connect to the webclient from within the mobile device browser? Just as a quick test?
Also this usually mean that 3CX Tunnel is disabled for the extension. Are all the extensions experiencing this? Can you ask the administrator to check in Management Console to confirm this setting is Enabled?
Management Console > Users > Extension > Phone Provisioning > Select 3CX App in the Dropdown menu > Use 3CX Tunnel for remote connections (3CX App only)