Hi @Maaike1, welcome to our community! 
The Xperia answering machine is not available in the Xperia XA1 Plus, so the voicemail you are currently using is the one provided by your operator. This can not be disabled in the device, but your operator should be able to disable it on your subscription, so get in touch with them and hopefully they can assist you.
Have a nice weekend and let me know how it went!
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