What you need is not to type up a letter but an affidavit, stating you want a paternity test and the mother does not, and have it signed by a public notary. Has this woman filed a paternity action against you? If the harrasing calls continue you have the option of reporting it to the police. If YOU really want to know if this is YOUR child, I would highly recommend you, if the mother has not done it, to file a Petition for the Determination of Paternity and Related Relief in the judicial circuit of your county court. You will file the affidavit at the same time you file the petition.
You can:
1. Hire a lawyer to do everything for you.
2. Get down to work, learn the Law, and file yourself.
Best,
UnwedDad