I have recently faced an identical problem where I need to modify the calling number by deleting the first digit of the number.Maybe you can similarly workout something to fix your issue.
Procedure for removing "0" from string "0939676" is as follows.
1. PhoneNumber is a string variable with a value "0939676".
2. Define X as a string variable.
3. Use "Execute Java Method" step to remove "0" from the string variable PhoneNumber and
store the resultant string value in "X".
4. Set the properties of "Execute Java Method" as mentioned below...
4.a. Select Variable : PhoneNumber
4.b. Method : substring(int)
4.c. Argument#0 : 1
4.d. Assign To Variable : X
Note : Value specified in argument#0 determines the position from where the string has to
be extracted .