There is no maximum length for voicemail drops but there is a 20MB file size limit. To give a bit of context, a 3-4 minute audio file depending on quality is between 5-10MB.
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​Ring.io Pro Tip: Keep your voicemail drop recordings between 15-30 seconds! Most people don't want to listen to a voicemail that's longer than 30 seconds. Keep it short and to the point!