The interference could be in UL / DL or both. if its in the UL you should check the Interference on Idle stats if in DL check the Drive test and in it any particular TRX (on which call quality is degraded) /frequency which might be effected.
Also if its interference then your UL / DL Interference HOs should be degraded.
You can also check be disabling the hopping on the cell and analyse the TRX level KPI to filter if any one TRX is faulty or a specific frequency or all frequencies.
Then devise a plan accordingly .. for external interferences below suggestions are the best but if its in a specific band and you cannot get the jammer off then try using the site of another band like if interference is in GSM convert site to pure DCS.