Not really a Firefox question.

Cell phone jammers are illegal or prohibited in many places (in US and Canada for example with fines) even though the employer may just be trying to keep the employees from taking calls or texting on their phones during work hours.

There are articles around on some possible ways around the jammers if you need to make a call depending on the connection options you have with your phone. However if the employer is resorting to this then you may risk your employment if you use your phone if not allowed during work hours.

Unless you want to lose your job, No.