Umbelieavable

Does anybody say um-believable instead of un-believable?
What about ing-credible instead of in-credible?

Any dialects? How common?

I think I say all 4 of those. I don't think I have a dialect that can be defined or is immediately identifiable.

It's much easier to pronounce an "m" (a bilabial nasal) instead of an "n" before a "b", which is also bilabial. A similar process occurs when an "n" is followed by a "k" - the "n" becomes a velar nasal because of the following velar sound (=k). It's a natural process in (faster, colloquial) speech but I don't think it's dialectal.

I've never heard it pronounced it like that.

But then why do dictionaries use /n/ in transcriptions? I guess you would end up saying um-believable in fast speech, unconsciously, but you would never say um-believable when speaking slowly. Right?
Then it would be like saying "unbelievable" can be pronounced um-buh-luh-vuh-buh. You don't say it that way in slow speech, but it would come out that way when you are talking too fast and you are too drunk.
So I guess saying um-believable would not be something to teach to ESL learners who want to reduce their accent... What do you think?

<<But then why do dictionaries use /n/ in transcriptions?>>

Dictionaries use broad transcriptions, therefore they don't indicate nuances like that.

<<I guess you would end up saying um-believable in fast speech, unconsciously, but you would never say um-believable when speaking slowly. Right?>>

Un-believable sounds a wee bit stilted and overenunciated to me.

I think I say something between un and um in unbelievable. I'm pretty sure I don't say "ing-credible"; that sounds really weird to me.