Need to join meetings with any mobile phone, Architecture & Lync-services?

Hello,

I'm done few basic Lync-implementations (IM, presence..) and I am familiar with Planning Tool. But, still I would like to know exactly what I need to implement to make it possible to join meetings/conferences with mobile phone, for example?

I mean that there would be represented a number to call-in in a meeting invitation, even from outside organization.

Lync Dial-In service maybe and PSTN maybe with a phone line or is even operator needed??

Thanks in advance,

RJ