Keyless door locks

[Generic]

This is the https://www.amazon.com/gp/product/B0097GT3P6 model we use. We have one on each room and one on a bathroom door. The one on the bathroom door gets the same code as one of our rooms. That way only those guests can use that bathroom.
We also have digital locks on rooms we don't want guests to have access to.
When a guest arrives, we write their room code on the back of one of our business cards. They need the code to get into their room and also into the house..

We give the code on a blank card. The business card has your address; if the guest loses the card, the finder has access to your premises.
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We give the code on a blank card. The business card has your address; if the guest loses the card, the finder has access to your premises.

And we don't write it down at all. However, some guests are very uncertain, so they grab a biz card and write the number on it. No matter how many times we say, 'it's YOUR phone number.'
Many tell me they don't know their own phone number, or their spouse's.
Whatever. Code is gone when they check out.
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We make them memorize it and won't let them write it. At worse, we will let them photograph it.