Is it permissible to use company tools (such as the phone and printer) for personal use, and what is the controlling factor in that, and does the ruling differ if the use is for da'wah purposes, or if the company is owned by non-Muslims?

The employee must safeguard the trust of his work by not using its tools except with explicit or customary permission. If their use is permitted by text or custom, there is no objection; otherwise, it is not permissible, even if it is little or for good deeds. What Omar ibn Abd al-Aziz did out of piety is required of whoever fears Allah regarding the Muslims' wealth. The Sharia (definition) is the scale for strictness and leniency.