I have read (sorry, no reference handy) that VoIP companies are not treated the same as landline and mobile phone providers in this respect. While they can take your number, they're not required to relinquish it. Seems odd but I can imagine it...
....but maybe that's all it is and what I thought I understood isn't true.
That is almost certainly true. I have read several porting FAQs that instruct to never cancel your number.
As another option, consider "remote call forwarding". You can do it *now*, it will give you lots of flexibility and there's not a huge cost involved.
RCF is provided by your local telco. From what I understand, it's usually marketed to businesses who want a telephone presence in a town (a local number) without having a physical presence there. When you get RCF service you specify a phone number where calls to some local number will ring. It *only* provides forwarding.
Not only is the service cheaper than regular phone service but because it's one way only many of the tariffs, etc. disappear. You'll also save on any special services you had (CallerID, distinctive ring, ...) because now you'll get them from your VoIP provider.
By going with RCF, you can choose whatever VoIP provider you want without being so tied to which one has a local number in your area. This is a big deal for me because I live in a DID black hole. (I forward my calls to a toll-free LiveVoIP number.) I also figure it will make switching to another provider trivial. I would hate to have my number in the hands of a company that goes under. (Note that toll-free numbers are much easier to port. That's one reason I'm starting to use them.)
If you're thinking about weaning off of your old number anyway, RCF might be a quick and painless way to get started and shave quite a bit off your local bill.
--kyler