A Numpad's Knight Numbers
Python 2, 52 bytes
f=lambda n:n&lt;6or`n%100`in'18349276167294381'*f(n/10)

Checks that any two consecutive digits are in the string '18349276167294381'. To get consecutive digits, rather than doing zip(`n`,`n`[1:]), the function repeatedly checks the last two digits and removes the last digit.