With IP Phone login, are you referring to Extension Mobility or Finesse IPPA. If you are referring to Finesse IPPA, then an agent at a given point of time can either use that or Finesse Desktop to login and take calls.
If you are referring to Extension Mobility then they will first need to the phone and then only they will be able to login into Finesse Desktop. In nutshell, in either of the case they will need to login independently.
Regards
Deepak