So looking at my inward dialing modifcation. We receive 4 digit DID from telco and in modifying the destination we do the following to add "21" to make it complete 6 digit.
Digits to Match: 8xxx
Digit Length Operator: undefined
Digit Length: 4
Number of digits to absorb: 0
Digits to be inserted: 21
So assuming that the SIP inward dialing modification uses the same rules, then as Ralph said- analyze the entire number and use "x" as placeholder.
Digits to Match: 44xxxxxxxxxx
Digit Length Operator: undefined
Digit Length: 12
Number of digits to absorb: 2
Digits to be inserted: 0