I went through this last month when I switched to the Nexus 5, had no clue I wasn't getting messages until someone with an iPhone tweeted at me asking why I as ignoring them.
However, all it took from me was a call to Apple's customer service, I told them I had just switched off my iPhone and no longer got texts from people with iMessage and they immediately sent me to a tech that fixed the problem for me.
Have you been explaining it correctly when you call? All I said was "I had an iPhone until last week, switched to another phone but I'm still registered for iMessage"
Edit: According to my phone I called 1-800-692-7753 (Which is just 1-800-MY-APPLE) and my call took 8 mins 25 seconds total. Not too bad of an experience.
However, all it took from me was a call to Apple's customer service, I told them I had just switched off my iPhone and no longer got texts from people with iMessage and they immediately sent me to a tech that fixed the problem for me.
Have you been explaining it correctly when you call? All I said was "I had an iPhone until last week, switched to another phone but I'm still registered for iMessage"
Edit: According to my phone I called 1-800-692-7753 (Which is just 1-800-MY-APPLE) and my call took 8 mins 25 seconds total. Not too bad of an experience.