IF the hospital (as the entity with the claim against you, for whom the collections agency is working) actually enters into a payment plan or other agreement with you, then that agreement is enforceable--they have to accept payments according to the plan, for example.However, without an actual agreement between the creditor and the debtor, the creditor, or its representatives (such as the collection agency) may seek the full amount due from the debtor. The creditor does not have to accept a payment plan, and does not give up its right to proceed against you because you have made some partial payments.The key issue then, is whether there  is an actual agreement in place.