Justin Bieber's Maybe Mama -- Lawyers Have Gone Underground

Mariah Yeater's attorneys had no problem giving interviews when their client came forward claiming Justin Bieber was her baby daddy ... but now that TMZ has poked some serious holes in her credibility ... they've gone radio silent.

Matthew Pare (left) and Lance Rogers (right) appeared on HLN this week saying they have "credible evidence" that supports Yeater's claims ... though they wouldn't elaborate on what that is.

And now that it's starting to look like Yeater is a fraud (she originally pinned fatherhood on her ex, before claiming it was Bieber's) we haven't been able to get Pare or Roberts to take our calls.

We've tried office numbers, cell phones, and emails both yesterday and again this morning ... NOTHING.