thirdlane communication manager 8.3.1.2, blind transfer ##456 issue

for a tenant, I call from extension 250 to 251, on 251 phone, I do blind transfer to ext 456 by pressing ##456

the asterisk only receive ##4, or ##45, never get the whole string.

in above trace, the digit 6 missed, I already press fast enough, sometimes, only ##4 received...

/etc/asterisk/features.conf, the relevant 2 parameters as following, are they relevant ?

transferdigittimeout => 3       ; Number of seconds to wait between digits when transfering a call
 

featuredigittimeout = 1000      ; Max time (ms) between digits for
                                ; feature activation.  Default is 500

[featuremap]
blindxfer => ##                 ; Blind transfer
atxfer => #*                    ; Attended transfer
disconnect => #0                ; Disconnect
 

also, the pbx is a busy one with many phone calls, is this relevant? 

any help is much appreciated. thank you very much.

the system is a busy one, with many phone calls.