because you turned on airplane mode, both cellular data and network is off
in most cases when there is a roaming partner, even if you turned off data roaming
you will still be connected to the partner's cellular network

and voicemail is treated as incoming calls, voicemail is enabled by default,
if you didn't answer your call it will go into voicemail, they will treat it as you have accepted incoming call
some even allow incoming sms as trap

complicated,
when your telco detect your ip address is from oversea they will deactivate vowifi
when vowifi first introduced in malaysia, it's basically like that... end of story

now digi & umobile (if not mistaken) allows vowifi while roaming
i'm only familiar with digi, since i canceled my maxshit account & didn't try others

digi scenario
A: MY number in MY
B: MY number in SG
C: SG number
D: MY number in SG (your imaginary gf travelling with you)

1. A & B both way vowifi call, local rate or free if unlimited calls
2. B call (outgoing vowifi) C, idd rate for B, no charges for C
3. C call (incoming vowifi) B, idd rate for C, no charges for B
4. B & D both way vowifi call, local rate or free if unlimited calls

so as long as you are in unlimited calls plan, vowifi calls to & from any MY numbers is free for digi
roaming outgoing and incoming rate don't apply, if it doesn't fallback to cellular network