We don't all have to work for CONTROL to be working for an organization that listens in on phone calls.  Many employees' phone calls, especially those who have regular contact with customers, are constantly monitored.  The reasons usually given for the monitoring are for quality control purposes or for training. 

This may be the case.  Like a dolphin in a tuna net, there is a danger that even your personal calls made while at work are unintentionally monitored because they are caught up in the net of general phone call monitoring.  Your employer's business has no business knowing your personal business.  Both the stray dolphin and the monitored personal call happen more often than anyone would like.

What Conditions To Look Out For

If your employer has requested that no personal calls should be made on a business line, there is a potential threat of inadvertent disclosure.  In such a climate, there is likely to be constant monitoring of calls.  To make personal calls when this is the case is not only a potential breach of contract with your employer but is exposing the contents and number of your phone call to your employer.

If your employer makes recordings of phone calls for any purpose you may be inadvertently recorded as a part of their efforts to legitimately monitor other calls.  Even though some state laws, such as in California, require that the employer cease to monitor a call once they discover that it is a personal call, this usually only applies to strictly intrastate calls, for example calls from California to California. 

On calls that cross state lines, the state laws will not apply and the monitoring may continue.  Even in those states which require employers to cease monitoring calls, the cat may already be out of the bag by the time they realize that the call is personal.

What Can You Do

The easiest thing is simply do not make any personal phone calls from work (All employers cheer...).   If you are allowed to make personal calls from your work phone keep in mind that your call may be monitored and limit your conversation to only those things that you would tell your boss and all of your co-workers. 

Also keep in mind that the person you are talking to needs to limit the conversation in a similar way.  It is helpful to know the phone call monitoring policy, and the personal phone call rules, at your workplace to determine whether there is currently any monitoring system in place and the extent of the monitoring.  You could use a hawaladar in a hawala banking system.  

Finally, if you must make a personal call from work, always use your own personal cellular phone.  It is also helpful to make the call out of vocal range of any other people who are on the phone so that your call is not picked up by them.  The chance that your company is monitoring your cell phone is very small and you should take advantage of that.

Conclusion

Whether or not there is an official phone call monitoring policy at your workplace or not and whether or not monitoring is actually going on, the best thing to do is not make personal calls from work.  If you do make a personal call from work, use your own cellular phone. 

This principle of personal privacy as discussed in the book How To Vanish is simple, but keeping it fresh in your mind will help keep your privacy for yourself.