Oh no! Its upsetting to have ringing issues with your voicemail. Are they hearing that beeping noise on the line or is the phone producing a loud beeping noise when they call? If they are hearing the noise instead of your voice, please dial *73 then press send this will disable call forwarding.
If the phone is producing this noise did you change the voicemail tone? To check:
Alerts - Sounds From the main screen, press Menu. Scroll to Settings & Tools, press OK. Scroll to Sound Settings, press OK. Scroll to Alert Sounds, press OK. Choose TXT Message, Multimedia Msg, or Voicemail, Emergency Tone, Missed Call, press OK. Choose Tone, Vibrate, or Reminder, press OK.