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I don't believe you can extend the amount of time a Voicemail can be saved unfortunately.
Have a read of this similar thread about trying to retain voicemails.
This is from a thread a few years ago.
" All you'd need to do is get a cable which plugs into the headphone socket on your phone, and then connects to the microphone socket on your computer (both tend to be a 3.5mm jack socket now and you can get one of those cables for a few quid online). You'd then need to use sound recording software on the computer, dial the voicemail number and record the message as it plays into your PC. I can't recommend any software, but many computers come bundled with such software, or there are free versions available to download. "
Current Phone > Samsung Note 10+ 5G _256Gb > Model: SM-N976B.
Samsung One Ui 2.1 / Android 10.
Samsung Gear s3 Frontier Watch.
Samsung Galaxy Buds.
If this is for important evidence, the Police or solicitor should be helping you with this, and retain the voicemail recording on file, they should know the voicemail would be automatically deleted or the voicemail evidence may accidentally be deleted and lost.
Try this link where there is an explanation here