iphone 4s - texting off 2 diff numbers

Hi - my son's new iphone 4s is being weird - he took his old number from O2 to EE but when I text his old number (off my iphone 4s), his reply is from a number I don't recognise. When I text his old number it goes as a text message, if I send it to this 'new' unknown number it goes as an imessage. Confused - any advice please... I can't get through to EE to get any sense out of them.



Thank you.

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