AT&T, T-Mobile, O2 and most other providers delete messages from voicemail after a period of 30 days.
Some providers offer an extension of this period for a fee. Contact your provider for more information.
Where can I save my voice mails for extended periods? iphone only holds 20 voice mails then deletes the oldest.
I use decipher voice mail to save iphone voicemails. It's $5 and it lets you save voicemails to your computer as mp3s or wav files I think. http://decipher-media.com/iphone-tools/ Good luck.