In extensions.conf, how to prefix a digit to variable{EXTEN}

Asterisk Asterisk Support
1

I configured this:
exten => 604…,1,Dial(SIP/1${EXTEN}@VoipStunt)
expecting the string becomes 1604… however, it doesn’t work. the box still calls 604…
Please advise. Thanks.

2

got the solution:
exten => _604XXXXXXX,1,Dial(SIP/1${EXTEN}@VoipStunt)

I mixed the wildcards with those on Cisco gateways.