number is a necessary parameter (at least one parameter) for MT call on emulator, and the number will be examined via gsm_check_number which only allows 0 .. 9, + and #.
And,
"gsm cancel" need it to hangup the call on MT side.

Besides,
the result we execute gsm call <SP> will be:
gsm call 
KO: missing argument, try 'gsm call <phonenumber> [<numPresentation> [<name> [<namePresentation]]]'
gsm call sp
KO: bad phone number format, use digits, # and + only