How can I connect a wireless filter to an old rotary phone?

You may need to post more details. Does the phone lack a modular plug?

The phone does not have a modular plug

I agree more details are necessary. What exactly are you trying to do.

I you are wanting to connect a modular device to an old hard wired phone, the easiest thing to do is upgrade the phone and its junction box to modular. Buy a modular cord and a modular junction box. Cut one end off the cord. Open up the bottom of phone and disconnect the old cord making sure you can tell what colors go where. Often things are marked R, G, Y, and B for red, green, yellow, and black. Connect the wires from the new cord to the correct terminals. Remove the old junction box and connect the wires to the new box. You can now plug in any modular device.