AASTRA DT4x2 for BusinessPhone User Guide EN - Manual (Page 8)

Important
BusinessPhone – Cordless DT412/DT422/DT432 8
Cleaning
Clean the Cordless only with a soft, water dampened cloth. Using
soap or any other cleaning materials may discolor or damage the
Cordless. Clean the battery contacts using ethanol or isopropyl
alcohol.
Treatment
Although the Cordless phones are resistant to water spray, dust
and soil, you should always handle it with care. Do not expose the
Cordless to direct sunlight for long periods and keep it away from
excessive heat and moisture.