"Best" is not so much the concept here as, "Works for you." There aren't companies I am aware of that do solely phone shops or online shops. Rather these kinds of shops pop up with companies I am already working for and sometimes they are on the job board of the company and sometimes I am contacted by email to see if I am interested. Your best bet is to register with a number of companies and perform shops for them such that if/when they have phone or online jobs they will offer them to you based on your overall reliability.