Here are a few tips:
To speak clearly, use complete words, like "Yes," instead of "Yeah" or "Mmm-hmmm."
Repeat a question before you answer it. This helps to clarify that you understand a person's question before you answer the question.
Example:
**Other person says, "Do you have any clips?"
**Then, you repeat their question to clarify, "Are you looking for paper clips?"
**This will help them clarify,
"Yes,"
or...
"No, I'm looking for hair clips."
or..
"No, I'm looking for CHIPS."
Use phrases like "please" and "thank you." This helps you to appear friendly and polite.
If you must put a person on hold, don't just say, "hold on," or "hang on," say:
"May I put you on hold for a moment?" or
"Can you hold, please?"
Smile while you speak. This can be heard in your voice, and people will be more patient with you if you sound happy.
Allow the other person to stop speaking before you start speaking. If you must interrupt, do so by saying, "Ma'am?" or "Sir?" before you start speaking over the other person.
Practice a lot. Speak on the phone as much as you can. Also, when you have free time, try to listen to other people speaking on the phone, as long as they don't mind you doing so. I've learned a lot about communication just by observing other people at work speaking with clients.
If you are still in need of better communication skills, I suggest taking a speech/communications class. This can help in many situations, phone or otherwise.