Got it. I googled what is USSD code. I tried *126#, the line is not activated and need to dial 016 2211800 to activate it.
According to that Community link I gave earlier, the line can only be activated using their local network...
If you really want to do it, can try somewhere near the causeway if you're not going over yet. I've tried places like the Kopitiam in Kranji MRT Station, but can't remember which networks, though.
Can also try Woodlands jetty (postal code 757446), or HDB block nearby (postal code 730215), or areas around there.
The last time I visit there -- Woodlands jetty -- I can get Maxis, Celcom & Digi signals on my phone.
I wonder if -- people who lives in HDB near Woodlands jetty need to be (always) careful when making & receiving calls on their phone. Can easily connect to M'sia telcos on their phone & end up making or receiving roaming calls on their phones.