Disclaimers first, I cannot offer all options in the universe but I can only share with you what I did to get mobile data and local phone number for the time being while I shop for more permanent mobile phone plans.
Coming from Singapore, I am a subcriber of M1 mobile phone plan with 3GB local (Singapore) data plan. I did not cancel nor suspend my M1 line because I still need the number for many things – such as to get OTPs for government websites that require Singpass access, online banking and of course the friends / contacts that you have established for so long using the Singapore number. So, I subscribed to M1 data passport, which allows me to pay just additional S$10 per month to be able to use my 3GB local data plan in HK. I love this data passport. It is really awesome. Connection works well mostly so far. It allows me to have mobile data wherever I go in HK, which is very essentially in this day and time. But I still did not have a local HK number which was also very necessary given I am to live in HK.
So, I got a dual sim phone – I chose samsung s7 edge. You can choose other cheaper dual sim phones too but they are typically Android phones – I am sorry if you are a die-hard iPhone fan. Then I got a prepaid CSL sim card at the airport (you can get cheaper ones at Sham Shui Po too, but I will leave Sham Shui Po to another post to avoid this post becoming overly long). Prepaid SIM in HK, not those tourist ones that can only last 7 to 14 days, can last for 6 months so you can use this prepaid SIM number for a while for various things such as, for your office contact, HKID application, rental etc, while you slowly settle down and shop for a HK mobile phone plan.
With the dual sim phone, Singapore SIM card with M1 data passport activated, and the prepaid HK CSL sim card, you can get to use mobile data like you always do in Singapore while still being able to call your office, make reservations, call to enquire anything in HK without having to bring around 2 phones and tethering your data etc or making commitments to mobile phone plans when you are not ready yet or breaking a bank (bcuz the prepaid 4G data here in HK is pretty expensive)
Yup, there you have it, an alternative option to keep yourself connected in HK while you take your time to consider your options. Do remember to get your HK phone number with the banks / authorities in HK updated when you change to the more permanent one though.