Q: Ive always had voicemail on my iphone 3s and now it has disappeared.  no one can leave a message and i can find where to fix it on my phone

I have an ipone 3s and always had a voicemail on it.  The voicemail has dissappeard and I cant figure out how to add it back on.  When someone calls it rings and rings and then after about 15 rings it says, unavailable, call back later.  I dont know how to reset it.  i have tried to reset network settings and that didnt work either.

If anyone know how to fix this, it would be great.

Thank you

Sandy