asterisk users - Jul 2016 - Please help me understand lines and extensions little better

Hello,

Probably obvious to most of you but I?m still learning VOIP concept(and
telephony inside organization in general)

I have bunch of unrelated questions on my mind :)

- I picked SPA504G phones as ?phone to go? for our small company. They are cheap
and available. Any pros/cons you can come up with against this device?

I setup direct extensions in Asterisk, they work great. But under conference
settings - I have to set which SIP devices participate. It just sends calls to
those devices and they come in as main extension.
It?s not super-convenient. Main problem is - I want to know if user came from
SALES/SUPPORT or DIRECT line.

-Now I?m getting more into phone setup little more and I think I?m not doing it
right.
Current on a phone I setup only 1 extensions with SIP ID/Password and then I
assign this extension 1 to all 4 ?Line keys?

I understand this is how I can switch between calls. Let?s say one call comes in
- I can be on call and next call comes in - I can press ?Line 2? button and
pickup. Correct?

So, now I got idea about Line key 3 & 4.
If my main SIP ID 
XXX

I can set 2 more id?s:
XXX_SLS
XXX_SPT

Then I can register them under ext 3/4 on a phone and set line keys to ext 3/4
Of course, I will add XXX_SLS and XXX_SPT under appropriate queues.conf

This way I will setup more extensions but users will have much better visibility
of who?s calling.

Is this correct/common approach? Just trying to understand what is the best way
to setup phones/extensions..