The tone of the voice was warm, lyrical, intense; it swirled and flared like a flame and filled the air with melodious reverberations like a carillon. It was by any standards an amazing phenomenon, or rather it was a great talent that needed control, technical training and strict discipline in order to shine with all its brilliance.
Trivella agreed to tutor Callas completely, waiving her tuition fees, but no sooner had Callas started her formal lessons and vocal exercises than Trivella began to feel that Callas was not a contralto, as she had been told, but a dramatic soprano. Subsequently, they began working on raising the tessitura of her voice and to lighten its timbre. Trivella recalled Callas as: A model student. Fanatical, uncompromising, dedicated to her studies heart and soul. Her progress was phenomenal. She studied five or six hours a day. ...Within six months, she was singing the most difficult arias in the international opera repertoire with the utmost musicality.
On April 11, 1938, in her public debut, Callas ended the recital of Trivella's class at the Parnassos music hall with a duet from Tosca. Callas recalled that Trivella: had a French method, which was placing the voice in the nose, rather nasal... and I had the problem of not having low chest tones, which is essential in bel canto... And that's where I learned my chest tones. However, when interviewed by Pierre Desgraupes (fr) on the French program L'invitée du dimanche, Callas attributed the development of her chest voice not to Trivella, but to her next teacher, the Spanish coloratura soprano Elvira de Hidalgo. Callas studied with Trivella for two years before her mother secured another audition at the Athens Conservatoire with de Hidalgo. Callas auditioned with "Ocean, Thou Mighty Monster" from Weber's Oberon. De Hidalgo recalled hearing "tempestuous, extravagant cascades of sounds, as yet uncontrolled but full of drama and emotion." She agreed to take her as a pupil immediately, but Callas's mother asked de Hidalgo to wait for a year, as Callas would be graduating from the National Conservatoire and could begin working. On April 2, 1939, Callas undertook the part of Santuzza in a student production of Mascagni's Cavalleria rusticana at the Greek National Opera at the Olympia Theatre, and in the fall of the same year she enrolled at the Athens Conservatoire in Elvira de Hidalgo's class. In 1968, Callas told Lord Harewood, De Hildalgo had the real great training, maybe even the last real training of the real bel canto. As a young girl—thirteen years old—I was immediately thrown into her arms, meaning that I learned the secrets, the ways of this bel canto, which of course as you well know, is not just beautiful singing. It is a very hard training; it is a sort of a strait-jacket that you're supposed to put on, whether you like it or not. You have to learn to read, to write, to form your sentences, how far you can go, fall, hurt yourself, put yourself back on your feet continuously. De Hidalgo had one method, which was the real bel canto way, where no matter how heavy a voice, it should always be kept light, it should always be worked on in a flexible way, never to weigh it down. It is a method of keeping the voice light and flexible and pushing the instrument into a certain zone where it might not be too large in sound, but penetrating. And teaching the scales, trills, all the bel canto embellishments, which is a whole vast language of its own. De Hidalgo later recalled Callas as "a phenomenon... She would listen to all my students, sopranos, mezzos, tenors... She could do it all."[24] Callas herself said that she would go to "the conservatoire at 10 in the morning and leave with the last pupil ... devouring music" for 10 hours a day. When asked by her teacher why she did this, her answer was that even "with the least talented pupil, he can teach you something that you, the most talented, might not be able to do."