So, long story short, I'm meeting up with a Japanese person who has come to the UK. Simply texting the number I was given bounces the message right back, however, and the same thing happens when I text with the Japanese international dialling codes as well (0081 I think, since I'm in the UK). I'm not sure if I have the wrong codes or not, since I haven't been able to find a completely clear explanation, especially one that applies to mobile/cellphones rather than landlines.