To keep your Heya mobile line alive, you have to (i.e. "forced" to)
spend money to buy any Heya plan every 90 days.
(If you want to enjoy rollover, then you have to buy another Heya plan every 30days -- before your current Heya plan expires.)
See Heya FAQ #8 under 'Manage account' -- at this link below:
https://heya.sg/support
8. What should I do if I want to terminate my heya plan?
Your heya SIM and all services will be automatically terminated if you do not top up with any heya plan within 90 days from last top up date.
PLUS, the
FREE incoming calls on Heya line is ONLY available from Day 1 to Day 60 (since last purchase of any Heya plan).
FREE incoming calls on Heya line -- is NOT AVAILABLE from Day 61 to Day 90 !!
Anytime -- within 90days --- when you spend money to buy any Heya plan, things reset back to Day 1.
**
M1 Maxx base plan is also $0 (until further notice) -- and comes w/ FREE incoming calls + 200MB local data + 10mins outgoing local calls + 50 local SMS every month.
But
M1 Maxx does not have "forced" spending -- in order to keep the M1 Maxx line alive.
To keep the M1 Maxx line alive -- just have some usage (by utilising the M1 Maxx base plan), i.e. make a call, send a SMS or use mobile data -- every 90 days.
**
If wish to keep a line just for incoming calls -- M1 Maxx is obviously the better option.
(i.e. keep a Heya line just for incoming calls makes no sense -- since the better option M1 Maxx is available)