My iPhone 5s won’t receive iMessage from some devices

As far as my knowledge goes the iMessage works in the following manner.

• Phone No. P1, Network Carrier C1, Device D1 (iPhone)
• Phone No. P2, Network Carrier C2, Device D2 (iPad with Cellular Service)
• Device D3 (Mac)

Following assumptions

All devices D1, D2 & D3 uses the same Apple ID

D1 > Settings > Messages > Text Message Forward > [Both D2 & D3 are enabled]

D1 > Settings > Messages > Send & Receive > P1 is Selected

D2 > Settings > Messages > Text Message Forward > [Both D1 & D3 are enabled]

D2 > Settings > Messages > Send & Receive > P2 is Selected

D3 > Messages > Preferences > iMessage > Start New Conversation from > P1 is Selected

You can send and receive iMessages as follows

• Send from P1 using D1 to any other iOS device across the globe. (P1 must remain in the same carrier network of C1 or have an international roaming enabled using P1)
• Send from P2 using D2 to any other iOS device across the globe. (P2 must remain in the same carrier network of C2 or have an international roaming enabled using P2)
• Send from P1 using D3 to any other iOS device across the globe. (P1 must remain the same carrier network of C1 or have an international roaming enabled using P1)
• Device D3 can send and receive iMessage using P1 being anywhere in the globe under any WiFi network but (P1 must remain the same carrier network or have an international roaming enabled using P1)
• Can send iMessage using Apple ID anytime anywhere any device

In a nut shell

If I have a Phone Number from India on my iPhone and I travel to USA without international roaming enabled., my iPhone will seize to send iMessages using the number I had.

But If I leave the iPhone in India (Powered) and travel with my Mac to USA, I can continue to send and receive iMessages / SMS using the same phone number I had, from USA. The recipients will receive as though it is being sent from my Phone number.

Hope, I have been able to make things clear.