Well, any button's LED can be configured to display state of so-called subscription. In most common case, the subscription is tied to state of particular device, but it can be anything else. The phone doesn't care the source of state nor how it is derived and from where.
So as long as your exchange can manage subscription according your needs (e.g. exchange will change subscription state accordingly if 101 is calling you, will change state of other subscription accordingly if 102 is calling you) then you can attach button's state to those subscriptions and they will behave according your needs.
It's rather simple task with Asterisk exchange, I know nothing about others.
Note you asked for "signalling". If you want to use the button to pick up the comming call, then it is more complex task and may not be possible at all. Althougth Cisco can configure both blf (with sub= attribute) and cp (with ext= attribute) functions on the one button (as fnc=blf+cp), it doesn't work as expected. There are unclear interactions betwen sub= and ext= ...
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