Reservations should be plural. if you're talking about how you use your voice to express yourself it should be 'tone of voice' and not 'voice tone'. They are interpreted slightly differently. 'Voice tone' is generally how your voice sounds, whether it is high or low pitched etc. You can't really change your voice tone. If you say they had reservations about your 'voice tone' a native speaker would probably interpret that as you having a strange or unusual sounding voice. I doubt that is what you wanted to say. 'Tone of voice' on the other hand is how you say words, your intonation, your word order, the rhythm of your speech, the speed of your speech etc. You can alter this in a foreign language with practise. I think this is probably what you meant. So the phrase would be 'had reservations about my tone of voice'.