First of all, this collection agency-Apex financial breaks all of the laws according to the FDCPA (Fair Debt Collections Practices Act). Their collectors do not give the mini-miranda-"this is an attempt to collect a debt, any information obtained will be used for that purpose". This statement has to be given to the person they are calling at the beginning of the conversation, if they do not give it, they are breaking the law according to FDCPA, and if you contact an Attorney, you can sue their company for at least $100 for each time they do not give the mini-miranda.  Second, if they contact you at your place of employment, or on your cell phone, all you need to do is verbally cease them by literally telling them not to call you work or your cell phone. When it comes to letters from them in the mail, or calls to your home number--you can fax them, or write them at
Apex Financial Management
1120 Lake Cook Road
Suite A
Buffalo Grove, IL 60089
866-876-2739 Toll Free
******847-947-7094 Fax********
letting them know that you are sending them a legal written cease, and that they are not to contact you by phone or fax. If you send the written cease, you should send it certified with delivery confirmation (so they cant pretend they never received it) and if you fax it, you should literally fax it 5-10 times over the period of one or two days,so again they cannot pretend they never received it.  If they do not abide by these ceases, you can sue their company and 100% of the time be awarded by the court up to $10,000.
This collection agency buys debts from creditors, making it incredible difficult for them to actually collect the debts, which is why they  regularly break so many laws. DO NOT let them bully you. They are all talk and easily dealt with if you follow the above referenced instructions. Good Luck!