iMessage Deactivation Timeframe
I am travelling with two iPhones: an older travel iPhone, and an in use iPhone. The travel iPhone has my home (Singapore) sim card with mobile data turned off, and the in use iPhone has a local sim card (UK). The travel iPhone only connects to the network via hotspots or wi-fi.
I’ve configured iMessage to receive messages at my Apple ID, the Singapore phone number, and the UK phone number; and to send messages from the Singapore phone number only.
My issue is that after an indeterminate amount of time—sometimes as little as a few hours—the Singapore phone number disappears from iMessage and each time my travel iPhone connects to the network, it has to go through the iMessage activation process.
Is there anyway to prevent iMessage from deactivating in this manner?
Update (via @AppleSupport): iMessage will remain active as long as your device has a reliable connection to the network. If it remains in a state where it is not connected to Wi-Fi, a hotspot, or the cell network, iMessage will deactivate.