http://revach.net/article.php?id=2234

Section: Halacha   Category: Tshuvos
Rav Shlomo Zalman Auerbach: Making Private Phone Calls From Work
If a person is an employee in a [government] office may he make private calls from his office phone?  Rav Shlomo Zalman is quote as saying (Aleihu Lo Yibol CM 28) that it is permissible to make two or three private calls a day because there is a basic understanding that a person will need to call home or other places during the course of a day.  It is clear that these calls should not be made in way that would hide them from one’s superiors.

It should be noted that Rav Shlomo Zalman was expressing his opinion of the norm in his place and time.

Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.