It would probably help to provide as much information as possible such as OS, version, service pack, system configuration, what version of the 3CX Assistant, and anything else you can think of.
This message appears when you have installed the Assistant without the VoIP phone but in the Assistant Preferences you have it set to use the VoIP phone for Dialing numbers.
You need to go to Preferences > Dialing > Make calls by PBX.
If you want to use the VoIP phone options you need to install the VoIP phone along with the assistant during the installation.