differnece between lync client call buttons

Hi I just want to know differnce between two these buttions. How to make it for an user in Lync client.

Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread. Thanks Chandru CT-HCL

ThanksChandru CT. MCITP