What outgoing caller id shows while porting numbers?

If you are only using the top level Company IVR menu (not using call queues) it doesn't matter.  

If you're using call queues, what I've done is add a new number to the call queue, made that the outgoing caller ID number, and kept it on for a time period of however long it takes to train persons from calling it.  In our case these numbers were exposed to customers and field staff for for over a month and we've kept them permanently.  I've not seen any other options.

TIP:  When setting up the porting process make sure you use the feature to assign temp phone numbers to the ported numbers.  This will automagically swap out the numbers and whatever device/queue that temp number is assigned to; ported number takes the place of the temp number and the temp number is surrendered back to RingCentral.  This work really well and requires no manual intervention to assign the ported number.  If you don't do this, then the ported-in number will be assigned to the top level IVR menu and assigning it to a queue requires a manual step.

Can't show a number that RC doesn't own [yet] - against the law down here in the 48.  So temporary numbers are your only choice.

Tom - when you say "automagically", I'm assuming you mean when the porting process is completed? 

Danno - pretty sure it's the same here "north of the 49th parallel".

If so, then it would make more sense to start the porting process and then only setup our current system to forward the calls when it gets closer to the date assigned in the porting process. Correct?