Every subscriber can have up to 10 alternate extensions that, if forwarded into Unity will go right to their greeting.
two digits is not supported by Unity out of the box but you can hack a registry key to force it to be allowed (TAC will not support this, however). If you go into the registry to HKLM\Software\Active Voice\Doh\1.00\
you should see a key called "Minimum Handler AVP_DTMF_ACESS_ID length" there - it's set to 3 - if you set it to 2 the SA will let you create 2 digit extensions for subscribers and call handler.
Be aware, however, that if you're using digital network with multiple servers doing this can cause addressing problems. If you're not using digital networking then this wont be a problem.