Searched here and on the internet and either the options have changed or my LG phone does not have the option. Seems like the old way to do it was to press the Overflow button while editing the contact, then select "all calls to voicemail".
Carrier is TMobile.
You should still be able to accomplish this through a different route.
being that you are using an LG on T-Mobile, I'll give you instructions from T-Mobile's website on how it's done on an LG G5.
Go to your phone settings → calls → decline and reject calls. Now press on the plus (+) symbol, and add the desired contacts.
Calls from numbers on the reject list, are automatically forwarded to your voicemail.
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