You can only have per-queue signatures. So if you are in need of
different signatures, please create different queues for that. You CAN
use user variables, such as <OTRS_CURRENT_UserFirstname>
<OTRS_CURRENT_UserLastname>
<OTRS_CURRENT_UserEmail> or even values from the user's preferences in
the signature.

So if the objective is to display the agent's phone number in the
signature, just create a field for the phone number in the users
preferences and use that in the signature. The benefit for this would
obviously also that users can update their own phone number and it's
reflected in the signatures immediately.