iPadio - http://www.ipadio.com

"My phone lightens my load" by Esther Gibbons
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iPadio.com allows you to set up a free Phonecast with a toll-free number that students can use to phone in their contributions. The "phlogs", as iPadio calls them, can be moderated so that they don't appear until approved.
Students could use the school phone during class time, and cell phones and land lines as homework, to call in their recordings.
Here are a few examples of how it could be used in the classroom:

iPadio in Education Several of the case studies include strategies for language learning.
iPadio Blog Posts tagged "education"

Google Voice - https://www.google.com/voice

Google voice doesn't offer a complete package like iPadio does, but it allows you to have a local phone number that students can call to leave a voice message. The number can be set up as a voice mail only solution, or set to ring on several phones at once. For a classroom number, it would be great to set up as a place for students to call or text, not only for homework recording, but also as a means to contact you when they need to communicate with you. Recordings are transcribed, and can be downloaded as .mp3's or embedded in a website. Google voice doesn't create a podcast, but it offers a free way for students to communicate in the language they are learning.

Using Cell Phones and Google Voice in Foreign Language by Liz Kolb, author of Toys to Tools: Connecting Student Cellphones to Education
Lisa Nielsen (aka: The Innovative Educator) shares some very compelling ways that Google Voice can be used by a teacher.
Everything Google Voice - from the Google Teacher's Academy