Can't be iMessages.. Maybe... Sort of...

So I've had several iPhones and recently purchased and iPhone Six and moved from O2 to vodafone. My pac code experienced serious problems being moved but that has appeared To be resolved. However tonight I noticed that my partner (who has an iPhone 5c) cannot imessage my mobile number, it sends as sms. However through some other people I've asked to do the same this problem does appear limited to just her phone. Although I can't 100% verify that.

i Have deleted myself as a contact in her phone, turned off every all the devices and hard reset them. But to no avail.

i Know this is a very odd issue, but I wondered if anyone might know what I'm missing or if this problem is her end or mine. Any input greatly appreciated.