Why won't my iPhone send iMessages to other iPhones?

I can use iMessage on my iphone4 with no problems at all to iPods, but not to my partners iphone4, it always defaults to SMS. Its frustrating, because we live in an area with poor mobile coverage, so if I need to message him urgently, iMessage in theory should be the best way of doing it. Does anyone know how I can alter this please?

You need to make sure iMessage is switched on, also Wi-Fi or mobile data, if it still doesn't work put their apple ID instead of phone number then it will send for deffo

Add her own email address in the Contacts section of the phone. Edit it so that her contact in the phone has both her number and email address and also make sure your Wi-Fi signal is strong and you have different email addresses.

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