Live-Chat setup on Wordpress website & mobile

General information for the website: TBA
Description of requirements/features: I need a Live-Chat message service setup on my Wordpress website of mine.. and my mobile.

I only require customer to admin chat (not customer to customer chat)

I want a FREE livechat messaging app, such as Tawk perhaps. But it must work well on both my website and mobile phone (iphone)

Please recommend to me the best FREE live-chat app that you think I should use?

QUESTIION... are live chat aps responsive also?

Immediate start.... (other work available if this goes well)
Extra notes: