One is that the modems are occupied by other applications, such as: Windows Fax Service, please close or turn off the applications/windows service(Fax Service) and try again.
The other is that the names of the modems contain a # character, you can right-clickMy Computer, click Properties, click the Hardware tab, and then click Device Manager, double-click the Modem branch to expand it, if the modems with a name containing a # character they should not be recognized by Joyfax Server. To fix this problem, try the following steps: Go to Control Panel and open Phone and Modem Options, click the Modems tab; Select a modem, and then click Remove, repeat the preceding steps to remove all modems; At last, install your modem drivers again.
If you want to install multiple modems of the same model on the computer, please refer to "How to install multiple fax modems?"
You can also Add Fax Device manually, for more please refer to: Configure Fax Devices