What does it mean when iMessage doesn't say delivered but the call goes through?

So I just got an iPhone 6 and I was texting a friend with the same phone. Suddenly my iMessages wouldn't go through. I tried calling them, and the call went through as normal and rung about five or six times but they didn't answer. When I touched the message I tried sending, one of the options that came up were "Send as text message". Does that mean the message wasn't even sent? I turned off my phone and turned it back on and it didn't say error. I'm just confused. I'm not sure if my message was even sent or not. I like to know how my phone works.

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