If you are an "individual user" (you are not working in an office that has a system administrator), then the problem is one of these:
You may have installed the product on a second computer. You actually cannot do this: the license allows you to install the product on only one computer. You must purchase additional copies of the product for additional computers.
You may have disabled the license on this computer because you migrated to another computer. You can no longer use the product on the first (old) computer.
User in an Enterprise
If you are in an office with a system administrator, speak to that person.
If you are the administrator and you are using a Group License key, this is what has happened: the person who got this message is on a computer (computer A) that was previously enabled for the license key but is now disabled. Perhaps you disabled computer A on the License Manager page because all your license seats were used up and you needed to let someone else install the product. In any event, here are some ideas for resolving the problem:
In your CounterPath Store Account, go to the Group License Manager page and disable a different computer (hopefully one that really is not being used) and then enable computer A.
Purchase more license seats and enable computer A.
The user should then be able to start the softphone without any problem.