Pretend to be happy when you speak. Whether you're frustrated or angry or sad or whatever, if you're letting that affect your mood, it will come out in your tone of voice.
If you try to be happy, or satisfied, or pleased, or whatever, you might be able to lift your tone of voice and make it more friendly.
Actors have lots of tricks for how to come across angry or friendly or sarcastic or frightened, but they have someone writing the words they want to say. Perhaps, if you have the time, you could write dialogue for yourself before going into an important conversation, so you'll have some idea of what you want to say, and some idea of how you want to say it?
Telemarketers (a dying breed, thank goodness) had the mantra "Smile and Dial." If you're smiling, it often comes through in your tone of voice as well.
I hope this helps.