That really bites when that happens , it's a game they play if this is the case. The old carrier has a legal right to keep your number for so many days before releasing it to the new carrier . Sometimes it's because they can't play in the sand box together and than there's human error .
The worst thing a customer can do is call and request their number to be released to the new carrier , they will stop your phone service at your request but than you'll have no phone for those days , you just gave them the right .
There are a couple of choices you have one is to call the old carrier and tell them to activate your number till it's released or the other is you can ask the new carrier to assign a different number to you so a caller can still use your original number but when you call out it will come up as the new number . This one can get messy depending on the experiance on the other end at the key board . Just putting it out there for ease of pain for anyone that might jump the gun while switching carriers , even though this might have nothing to do with your circumstance . Gotta love technology .