Here are some tips dealing with Collection Agencies. All communication should be in writing. Sending everything in writing provides you with documentation that you may need later on if you decide to report the collection agency. Insist, in writing, that the collection agency only deal with you in writing as well. Check the statute of limitation in your province or state, some debts have a certain time limit, after which the debt is no longer collectible. There can be no legal action taken against you after this time. Make sure you know the statute of limitation before you pay, because making a payment causes the statute of limitations to restart. This is why sometimes the agent may tell you to make a small payment to avoid "further proceedings".
Negotiate with the collection agency; they can always reduce the amount owed sometimes by as much as 70% or more. Some may also allow you to make a payment plan to pay off the debt. Do this in writing and keep all your receipts. Do not admit to debt you are not sure about, ask the collection agency to prove the debt and their right to collect. They have 30 days after the receipt to prove this. Again make sure it's in writing. You may ask the collection agency to cease communication. They will only be able to contact you to let you know that they are not pursuing your debt any more or that you are subject to legal action. Make sure you do this in writing.
These are just a few tips that I hope can help you when dealing with collection agents, in USA review the Fair Debt Collection Practices Act and understand it, this regulates debt collection agencies (not the original creditor). In Canada there are similar regulations put in place by your provinces, so check with your province for details.
Collector's can NOT threaten you in anyway, if you feel harassed or threatened by a collection agency file a complaint with your Consumer Affairs office. One final note: There are government guidelines that regulate collection agency behavior. It is important to have a correct understanding of your rights under the law.