If pilots are ever in any doubt or uncertainty they should ask - yes, it's annoying if the ATCO is busy and the frequency congested but it's a hell of a lot better than thinking you'll figure it out and busting through a control zone or not conforming to procedures correctly and making a busy arrival/departure/circuits sequence busy AND messy...
Also, if you say you're on frequency, LISTEN. Nothing more annoying than transmitting traffic or instructions to aircraft with NO response because they are busy yapping to their student/passenger/company freq etc....
