I recognize two errors in your code. First one is probably a typo: you are not surrounding the phone number with quotation marks so it’s an integer: 1 + 555 – 555 – 5555 = -5554
Also, you should use replaceFirst since there’s no method replace in String taking a Pattern as first parameter. This works:
def mphone = "1+555-555-5555"
result = mphone.replaceFirst(/^1/, "")