It is wise to find out from the original phone provider whether or not they have sold the debt off, If they have then you have no choice but to pay the agency. If they have not then you may go to the original phone provider and pay your account. Pay only the assigned original amount and they close the acct with the collection agency Interest and all.
In regards to a settlment... If it is found on your bureau as a settlement in full, in most cases if it's not satisfacory to the institution that is loaning you money they will request that you pay the remaining amount. However the occasion may arise that you contact the agency that you made a settlement with and they are unable to re activate the account. It can be a tricky situation. I hope this information helps you. Good luck!