At work probably 1 in 20 calls on my phone is actually for me.
I'm in a call group called Support and I only answer those if they ring for 1 minute or more (as that amount of time means the support team are busy).
As the majority of calls that ring on my phone are for support, when I'm engrossed in work and the phone rings, I don't even look at the phone screen unless it's rung for a minute or more, but by doing that, I miss internal/direct dial calls to my phone where people hang up after 10-15 seconds.
It would make things much better if direct calls to my phone (internal or external) had one ringtone and any calls to my phone from being in a group could have a different ringtone.
Being as my phone can display a number with the word 'support' or 'sales' next to it (which denotes which call group the call is in), I would hope that it is easy enough to make different ringtones for different types of calls.
I use a Polycom VVX500