I don't go with phone numbers on the keypads for security reasons. If people know that's what you're doing, they could look up someone's phone number and break into their room.
When people make an online reservation they have the opportunity to enter the 4 digits they want to use. If they don't enter anything, I assign a random code. In 15 months I've never had a single case of anybody forgetting their code or getting locked out. No worries with keys being lost or carried off. No worries with someone duplicating a key and breaking in in the future.
Stu,
the keypad locks I use hold up to 19 user codes at a time, so everybody has their own codes, including staff. My system lets me program the codes wirelessly from any computer or even my iPhone, and it lets me set the date & time the code will start to work, and the date & time it will no longer work. So I program it in advance and forget about it. Works very well..