Phone won't text certain numbers. Says they are invalid

[iMore Question]

My new iPhone 6 will allow me to text everyone except my dads phone. It kicks the message back saying it is an invalid number. I have deleted his contact and added back in with area code and it still will not recognize as a valid number. His phone is a Samsung Galaxy 5. Anyone else have this problem and how do I fix it? Thanks.

 

[MarkY3130]

Settings>imessage> toggle on send as sms


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