This MIB is used to configure digit maps for all endpoints of
the unit. A digit map is a sequence of digits that is
acceptable (or inacceptable) for calling. For example, the
digit map 'xx' will accept any two dialed numbers as a valid
digit map.

The permitted digit map syntax is taken from the core MGCP
specification, RFC2705: ftp://ftp.isi.edu/in-notes/rfc2705.txt,
section 3.4.

DigitMap = DigitString  / '(' DigitStringList ')'
DigitStringList = DigitString 0*( '|' DigitString )
DigitString = 1*(DigitStringElement)
DigitStringElement = DigitPosition ['.']
DigitPosition = DigitMapLetter / DigitMapRange
DigitMapLetter = DIGIT / '#' / '*' / 'A' / 'B' / 'C' / 'D' / 'T'
DigitMapRange =  'x' / '[' 1*DigitLetter ']'
DigitLetter ::= *((DIGIT '-' DIGIT ) / DigitMapLetter)

where 'x' means 'any digit' and '.' means 'any number of'.