Hong Kong's Blue Card is also quite convenient, essentially the same as the Greater Bay Area card I got from China Telecom in Macau. Some 7-Eleven stores sell it; I bought mine from an automatic vending machine near the entrance of West Kowloon High-Speed Rail Station (just a few steps from the Kowloon MTR station direction). The price is 100 HKD per card, and you only need to recharge 50 HKD every six months to keep the number active.

If you go to a designated location to upgrade to a dual-number card, you'll get both a domestic phone number and a Hong Kong phone number. Recharging to keep the number active is even cheaper—just 10 RMB within six months to top up the domestic number.
The convenience of this card is that it works across Mainland China, Hong Kong, and Macau. There are no fixed monthly fees, and the unused data package is valid until it runs out. The price is 288 HKD for 40GB, which also allows easy access to the internet outside of China.