Technically, you cannot use a mobile jammer for select calls alone.
It is not a good idea to fix mobile jammer in an office environment, the disadvantages of which has been elaborated by Rajkumar.
As Kambar has said, bringing in self discipline is the key.
Mine is a very small consultancy firm and have 3 employees. I don't restrict my employees from having personal conversation with their friends or family members, as long as their productivity is not affected. Staff members who have elders and children at home, tend to make calls and talk to them. I don't see any wrong in that. Even 5 or 10 minutes talk to a old friend, does no harm. Even in the midst of important discussion, I never stop my staff from taking call from their home or anywhere. We never know, what is the urgency of the call is.
Having said that, Don't we (that includes me too) expect our employees to pick official calls, when they are at home? When we expect them to handle official calls at home, we need to reciprocate by allowing them to take few personal calls at work. But a sense of self discipline should be brought out, as another member pointed out