In my case I had to try something different to make it work:
Symptoms:
- iPhone waitng and waiting for activation of iMessage
- Able to receive iMessage at my AppleID, but not my phonenumber
- No phone number listed when the phone was connected to iTunes
- Problems started after SIM change
Common heard resolutions that didn't help:
- Set phonenumber under settings->phone->My Number
- Turning iMessage on/off
- Turning the phone on/off
- Resetting network settings (Setting->General->reset->Reste Network Settings)
- Restoring the phone via iTunes
- Putting the SIM in another iPhone
What DID help:
Putting the SIM in a old (Nokia in my case) phone, and take a look at 'my phone number' in the addresbook (set the phone number if nessecary). This seems to update the phonenumber on the SIM, iMessage will activate fine hereafter.