The biggest feature I think many people would look for would be the ability to transfer an existing number over (it is a real PITA notifying everyone about a number change).
I'm guessing that there may be reasons why transferring a number in is not possible but I think it would be very useful if it was implemented.
Yes, there are a couple reasons.
One is complexity - we would need to develop tools to manage the porting process, and our goal was to launch quickly with this free feature.
The second is cost: porting a number not only results in a substantial up-front cost, but an ongoing directory fee that itself is about five times the base monthly the cost of a newly issued number. This directory fee is applicable whether the number is in the directory, or unlisted - it's a cost related to keeping that information either way.
We've recently conducted a survey of FaxLine beta users, and Kelsey did include questions about porting, and willingness to pay for porting. He may be better able to speak to the results of that in specific, but my recollection of the analysis was that zero of fifty respondents indicated they had a number they would want to port. However, when asked about paying for porting, while most skipped the question as N/A, five of the 50 did say that they would be willing to pay for porting (I guess, answering hypothetically, if they did have a number to port, though they indicated they did not in the previous question.)
As we wrap up debugging (thanks everyone for feedback and reports!), we will promote via email to current customers, with the current feature set (including new number assignment, w/o porting), then see what sort of demand we get from the full customer base. Based upon that, we will decide if it's something we want to invest the labor in, and we will determine if we can build an economic case around the costs that are sustainable.