rwbradley wrote:
... I suspect the number cannot be ported while it has another service on it... DSL. ....
BINGO! You are correct. Under the USA rules the carrier does not have to port the number if there is another service like DSL on the same line. I have been in this telephone business since 1970, first with Ma Bell then with a CLEC. We ran into this rule often.
To the OP, once you remove the DSL the local company is required to allow the number to port but they can refuse while there is a DSL active. The company you want to port to (like Vonage for example) has to do an inquiry before porting and that is why they tell you it can not be ported.
rwbradley has it correct.