Prince William Family Medicine Patient Resources. Forms, Billing, Insurance Accepted, Policies and More. Gainesville: 703-753-4999 Manassas: 703-257-8090. Gainesville: 703-753-4999 Manassas: 703-257-8090. Gainesville: 703-753-4999 Manassas: 703-257-8090. Prince William Family Medicine..