What is the cheapest prepaid SIM card for non-personal usage?

I am looking to get a separate number for handing out to work and non-close people.

Mainly to be used for WhatsApp, receive 2FA SMS code, and maybe some local calls (ideally free incoming, outgoing to be kept to a minimum).
Data is not a priority since my phone can support dual-SIM, and I'll be using my main line for the data.

Looked around and found:

• M1 $5 M Card (90 days): Minimum top-up of $10 for 120 days
• StarHub $15 (180 days): Minimum top-up of $10 for 120 days