20 second voicemail beep

This is a weird one....I even called Sprint and they have never heard of this before.

I have a recording which is around 28 seconds long for my voicemail. When I record the message, 20 seconds into it, I hear a beep. It continues to record all the message though. When I playback the voicemail greeting, the beep is gone, but there is a 1-2 second part of the message missing (a word gets cut off) and then resumes to play till the end. Any ideas why this is happening? Or better yet, how I can get around this? thanks!