[VoiceOps] SIP return message on misdials - 487 or 500/503 or ?

Hello, Just trying to get idea of what is normal on what providers should return for a misdial, bad unknown number, etc. On one provider, I get a CANCEL and a 487 Request Terminated on mis-dials. On another provider, I get a 503 Service Unavailable and a 500 Internal Service Error back. Is there a proper code to expect or is this up to the provider? I was working on chasing down another issue and noticed a lot of 500 and 503's on calls and they were all misdials (bad numbers, etc) on this one provider. I can't call these numbers from other systems or my cell phone so know they are not good numbers people are calling. Thanks, Erick