How long will iPhone voicemail messages remain in my voicemail box?

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. Good luck.