Try this method, it's worked for me after some trial and error.
Download a voice recorder app if you do not have one installed on your phone.
Open the app, then record a minute or two of silence.
Go to top right of screen and open the list of options.
Chose set as ringtone.
This gives you a choice of "ringtone", "contact ringtone", or" trim ringtone".
Then allocate the silent recording for the contact you choose.

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