I have an AsteriskNow system happily talking to VFX via SIP trunking, and an SPA3102 providing "lifeboat" PSTN calling both in and out. This is a home system, single SIP trunk, which I set up as a learning tool before setting up my work system.
Now to start figuring out the handling of multiple incoming calls.  I'll get there, but I thought it worth getting a head start by trying out the collective wisdom of the zoners :)

We will have 10 DDIs (for expansion), and 3 simultaneous call channels. ( 5 later)

Question the first:
   with the single trunk, I have one SIP channel definition [trunk_1] with the various VFX registration values, which sends incoming calls to the context DID_trunk_1 in the dialplan, and thusly to my IP phone.   ('fromuser' value = 995xxxx1, my VFX phone #)

With multiple DDIs, do I need to add a complete SIP definition for each, [trunk_2] ..[trunk_10], with the 'fromuser' being  995xxxx2 thru 995xxx10, or do all calls to the different DDIs come in to asterisk with a common 'fromuser' (or other value?)  and you determine the correct DDI target from something like the ${EXTEN} value?

My 2nd question :
  a common scenario will be calls coming into the 'main' number, 995xxxx1, and the receptionist answering and transferring them to other extensions.
Now - will VFX allow multiple calls to that one number, ( up to the limit of the purchased SIP trunks? (3 or 5) )
  ( I am hoping VFX won't care about multiple calls to one DDI number, just the total number for this account)