Once debt collectors are informed not to contact you at work, they cannot contact you there. The first time they contact you, tell them what number you want them to use for communication purposes and tell them to never contact you at work. If they are contacting you at work, tell them immediately to stop contacting you there.
Document the conversation
The key to dealing with debt collectors is keeping good records. If you are being contacted by one debt collector, chances are that others are contacting you too. Document the time, duration, and contents of each communication with a debt collector. Specifically, document the fact that you informed XYZ debt collector not to contact you at work anymore and document their response to you. Again, you will need to keep every agency separated so you can keep track which you've told not to call at work and which you haven't.
Get work done OR contact an attorney
Either the debt collector will honor your request and you will never be contacted again or they won't. If they continue to contact you at work you need to hire an attorney. The phone calls there could eventually cause you to lose your job if your employer finds out you're receiving personal phone calls. As such, nip it in the bud right away and contact a Fair Debt Collection Practices Act Lawyer.