Know why? Well, unless the receptionist was just talking to a person with approximately the same size vocal cavity as you, she or he has no hope of understanding you.
Vowel sounds are made by using the mouth as a resonating cavity, and distinguished from each other by such things as the position of the tongue, which changes the resonance. Since vocal cavities vary in size, we can understand a person's vowels only in relation to their other vowels. The pitch of a large man and a small child are very different. In fact, a larger person's high vowel in "see" could have the same pitch as a smaller person's low vowel in "saw."
Humans are amazingly good at understanding speech, and only need about two syllables to pick up a speaker's vocal range. This means that the first two syllables in a conversation will generally not be understood, and this is particularly true on the phone where the listener has no cue as to the speaker's size.
So give the receptionist a break. If you start by saying, "Hello," or even "Extension 432," you will suddenly find yourself much better understood.