Allow two and three digit numbers in callsign and allow nothing in the last box

[TEMPEST.114]

EG. Tail number of a/c 203. So callsign should be Enfield TWO ZERO THREE. 
 

The current call sign box only allows single digits in the first box and you can't remove the last, or set it to anything other than the 'UNIT INDEX' - this link is unfortunately unhelpful.