Voice Mail Doesn't Capture Messages

I've tried to test my voice mail. I called a wake up service to place a call to my Whistle phone number. The call comes in, and when I don't answer it, my Whistle phone rings 15 times, voice mail never answers. Instead I'm left with a missed call message on the phone. When I check my voice mail, inputting my voice mail pin number, I'm informed I have 0 messages? What's the problem? I have Whistle installed on my PC, Windows 10 OS. Everything I need is working, calls are going out & coming in fine, just no voice mail if I'm not at the computer to answer an incoming call, which is important to me. Thanks for any help!

(I might add that I did pay for premium service and the account has a decent balance on it for premium service purposes.)

As a new user, the instructions for setting up voice mail went by so fast I missed the last part telling me how to end the recording for the greeting. So, when I created a greeting, I didn't terminate it properly, and the greeting ran on and on with empty space. I wasn't sure when I created the greeting, having missed the step to terminate it properly, thinking to myself at the time, maybe it's voice activated and would terminate itself after I recorded the greeting. It didn't, so the greeting ran on for a long while with empty space. I think this caused a problem for the wake up call service I was using to leave a message in the voice mail box. I didn't know that till now.

Now that I can call in myself, I heard the extended greeting waited till it ended, left a voice mail message at the end for myself. Then, I called Whistle back, logged into voice mail, and there was my 1st stored message!