Your tone of voice is a key determinant in how others perceive you - it is just as important, if not more than the actual words you use!
Friendliness can be projected through the words used and the emphasis should be on using courteous, thoughtful, and caring language when engaging with other.
A "friendly" voice lets people know that they can trust, rely upon, and be reassured by you.
Consider how you can introduce a sense of a smile into your voice without necessarily having to smile.
Always keep a smile on your face - It's a wonderful add-on to your friendly tone.