VoWiFi is part of the HetNet features that all 3 telcos are working with IDA to roll out as part of Smart Nation initiative.
If you had studied about Mobile Comms, HetNet is one of the ways to offload mobile and data traffic off from regular cell towers (in a way its a good thing. Can help you save some battery power.

)
If your office is within that HetNet zone, you will be able to use the VoWiFi feature on your phone. If not, recommend that you turn it off to save some battery power. Squeeze another 3 to 5mins out of your battery also happy.
https://www.m1.com.sg/hetnet
https://www.singtel.com/about-Us/ne...island-wide-vowifi-rolout-and-paves-way-.html
Starhub haven't seen any HetNet trials yet. Next one should be MyRepublic, they are doing some VoWiFi trials at JLD area IIRC.
But when you guys say "I go office can use", you all sexpose roughly where you are working liao.

All JLD area.
But dun worry that this HetNet thing will jiak into your 4G or 3G Data consumption. The billing charge will count towards your voice minutes, not data. The background telco billing process can blow your brains out this early in the morning so I don't want to kill your brain cells.
Actually for that whole Smart Nation thing, it is a lot more complicated than everyone thinks it is. Unless you are very deep in the technology, government tech or security business, very little chance most people will truly understand the scale of Smart Nation.
Dun bother about your home router. It doesn't support HetNet... YET.