Reference: Fadhaa.il al-A’maal – Pages 7-8
Question: What is the ruling about the one who took something from his [place of] work – whether it was a little or a lot; and likewise, using the work telephone [for personal calls]?
Response: [Regarding] the work that is before you, then you are entrusted with it; Allaah (Subhaanahu wa Ta’aala) says:
So, it is from betrayal that you misappropriate anything which you have been entrusted with, even if the chairman [of the company] has permitted you. That is because quite possibly he doesn’t have actual ownership of anything [in the company]. If, however, he does have actual ownership, and is permitted to do so [by the company], then there is no harm in taking [what he grants you permission to take].