I can safely say it will be the same for M1. If u turn off your M1 line in SG, your M1 totally no connection to the network, how will it even know you are in overseas?
U do the call forwarding to a local SIMBA number, so ONLY local outgoing will incurred since u do the call forward when M1 out of service.
iPhone does not have the option to forward when out of service. It will only do ALL CALL FORWARD.
3 ways or 3 options for you.
1st - iPhone, you do a ALL call forward to SIMBA before u head overseas and turn off your M1 line. Once you are back in SG, u need to cancel the Call forward.
2nd - You find an Android phone, select Call forward when unreachable. This option means your calls will be forwarded when you are out of M1 reception or when you turn off your line/phone. Once you turn back ON your line or your phone and have reception, your calls will NOT be forwarded anymore. This forwarding can just keep it there since it does help when u did not realized that your M1 line no reception at certain places.
3rd - Last option, if u do not have any android phone or u are using esim for M1. U need to google for those call forwarding USSD code, example *71 91234567.. This way is similar to using an Android phone to do call forwarding when unreachable. The code above is just an example, please google for the actual code which I do not know off hand now. Good to share here once u found out.