HI!!! my bf experienced the same problem. he cant initiate sending an imessage but he can reply to the ones i send. so we headed on to our provider and inquired about it. rep said its a network issue but i wasnt satisfied, we headed out and i was still bothered. one thing i noticed from the setup of my bfs imessage compared to my setup was his contact number was added as one of the addresses where imessage will be recvd,it was graded out(unchangeable) and my numbr wasnt on my phone. i compared our setup and the prob was from my phone, my setup. I didnt properly activated my imessage, it was on but underneath says "activation unsuccesful." so what i did was switched of my imessage and after a minute switched it back on, now HEADS UP! the rep we talked to advsed us that if accidently imsg was switched off/on it sends an international message and the costs depends on your service provider. moving forward, after switching it on beneath that says "waiting for activation" and after a minute.. VOILA! my number was added to the adreses where imsg will be received. asked my bf to initiate imsg and he can now 😀 check with your friends/family if they have succesfully setup-ed their imsg. hope this helps!
*bumped into this thread thats why i came back and shared my resolution 😀