Instead of considering the installation of mobile jammers, though I am not sure if jammers can be used selectively to restrict outbound calls technically, I would recommend that you prevent the entry of mobile phones inside the office premises and restrict the entry at the security gates, which many companies do now.
The logic for banning mobile phones inside the workplace is that they could distract people working on the shop floor and could also pose a safety hazard to the employees. In some instances, you will find mobile phones ringing during meetings, disrupting the continuity of the meeting and causing attendees to lose precious time while waiting for their colleague to finish their conversation.
A question may arise about how an employee can communicate with their residence in case of an emergency and vice versa. The solution to this problem can be to permit employees to make calls after dialing a secret code. Upon dialing the secret code, they will gain dialing access. Thereafter, they can make either official calls or personal calls from any extension. The calls made, along with their duration against each employee, can be logged using the latest devices available. In the event that we observe an employee making calls to a particular number (personal call) numerous times, we can counsel the employee if they are found misusing the facility. On some occasions, there may be a family emergency that requires frequent calls to a residential number, which should not be considered a violation.
Regards