Any advice what will be the best & cheapest system?

I have a community centre where everyone can go in. They need to put a code to enter the centre. Usually people will call me to get the code.

However I want to setup a system where they can text / QR code / they'll get the code to the centre. Essentially i want to have an extra security perhaps the system will send the code if we have the mobile no or email no associated with the code retrieval.

Any idea? Suggestion? It's a charity org so cost is extremely important.