I recently moved from Sprint to Verizon Wireless. My first issue came on Day 1 when I needed to call to have the 4 numbers ported over. I told the rep that my son had the old phone until the evening and I wanted to hold off on his port until he was home. She said insisted that she could future-date the port for his number and it wouldn't go through until we called back in. The first three lines ported almost instantly. I noted that the My Verizon page showed a "pending order" for my son's line. An hour later, I called my son and couldn't get ahold of him. Instead, I get a message that "Voicemail has not been set up". His number had ported early. I tried for an hour to find someone in his area who could alert him.
When I called the Verizon Wireless porting department, I was told that I would have to re-establish service with Sprint, port the number to Sprint, then later re-port the number to Verizon Wireless. Needless to say, the Sprint rep was unwilling to help (which is part of the reason that I'm no longer with Sprint...)
There were a few moments of stress during the time my son's phone was unavailable. Thankfully nothing went wrong where he would have needed to contact us in case of emergency.
Bottom line: Don't count on schedule number ports, for fear that they happen too soon and you are left without service.