based on your predicaments, you just have to communicate through a letter or personally appear before the office of the internet provider. You need not discuss the matter to the law firm acting as collection agent of the internet provider to avoid unnecessary add up charges that will be given by the collection agent.
On the premise that you informed them to cut the services, if you can prove it much better. Presumably, you made a verbal coordination to cut the services, it would be remote considering that a denial would be the answer that you will get with your internet provider.
It is my suggestion that you settle the matter amicably. Formally write them a letter and state your predicament that the only way you can settle the arrears is by paying "xxx" amount reasonable for you to comply. It is also suggested that you issue post dated checks to guarantee you willingness to comply with the terms-just be faithful in funding the check.
Do not be afraid, some collection agencies are applying extra-ordinary techniques for them to threaten debtors and eventually pay unreasonable amount. There is a Bangko Sentral circular about this, I just forgot the series number but you can search the web, its all about the rules on collection to be made by banks and collection agents. this would apply in your case. try browsing the net on this, so you won't be threaten by undesirable collection agents out ther to threaten just to make collection. Also, in that circular, you will know where and what department of the Bangko Sentral you can direct a complaint against this agents/banks/business establishments (DTI for business establisments).