Just thought I might add what worked for me - maybe help a few people who are having the same problem... After going through the forums for days and trying everything I could to get my iMessage to activate on my mobile number, I restored it twice, hard reset, toggled all possible options and nothing worked... I was eventually convinced that it must be an Apple server problem.
Last night I logged out of my Apple ID on the phone, I switched off iMessage, I switched Cellular Data off and put my phone on Airplane mode (which is all a kind of combination I read in the forums). I went to sleep, this morning I switched Airplane mode off, switched Cellular Data back on, left it for about an hour and then turned my iMessage on, it activated almost instantaneously.
Whether there was a server problem which was coincidentally fixed before I turned my iMessage back on this morning, I don't know... but hopefully this could work for you, should you find yourself with the same problem.