You can also upload an unavailable greeting using the OnSIP app. After logging in, click on the Voicemail Settings gear icon at the far right. Make sure that the Custom Greeting is toggled to "on." You will then see an option to upload a .wav file of your choice. This recording will serve as your unavailable message.
It is as simple as that. Once you have left a message, document the call and the action that was taken in your client record. If you continue to only reach the client’s voicemail, after several failed attempts: What to Do. Send a collection letter. There could be a reason the client is not receiving or responding to your messages via phone.

Your research should include finding other people in the prospect’s organization. Selecting the right people, such as the prospect’s manager or department head, facilitates the ability to build rapport and adds a sense of urgency when you mention them in the voicemail.
Friends and colleagues speak to each using first names only. So do people of authority. They do not call each other and leave voicemail messages asking for Mister, Miss, or Mrs. Therefore, when you call a person you want to do business with and you leave a voicemail message, refer to them by their first name only. Don't say mister, miss, or misses. Don't say their last name. Begin your voicemail message by saying only "hi/hello" followed by the person's first name. Or, you can even forget the "hi/hello" and just say the person's first name. That is how you show confidence and authority and separate yourself from weak salespeople.