Devices, systems and methods are disclosed that allow a patient to self-treat a medical condition, such as migraine headache, by electrical non-invasive stimulation of a vagus nerve. The system comprises a handheld stimulator including a housing having a contact surface for contacting an outer skin surface of a patient. The housing is configured for attachment and electrical coupling to a mobile device configured to receive a wireless signal. An energy source is coupled to the stimulator and the mobile device. The energy source is configured to generate an electric field that transmits an electric current through the contact surface and the outer skin surface to modulate a nerve within the patient and thereby treat a medical condition of the patient.