An already Spoken to customer issue that has been resolved
Both are correct.
Notice the difference in tense.
"The problem is solved." [describes the current status of the problem]
"The problem has been solved." [directly answers the question Has the problem been solved?]
The difference is in whether you want to emphasize the state of being solved or the action of solving.
"is solved" indicates that it is in the solved state.
"has been solved" indicates that the solving of the problem has been completed, which is really just what it means to be in the state of being "solved".