lync 2013 with SIP trunk with panasonic kx-tde200

Hi

My company has installed a panasonic ip-pbx kx-tde for multiline with 100 number range for telephone service.

Now my company is going to replace multiline by sip trunk . It will still work with Panasonic pbx box just need to reprogramme to be able to connected to the sip proxy which is managed by internet service provider.

For this scenario , would Lync 2013 voice work if I just add PSTN gateway which is the ip of panasonic pbx address to the frontend in topology ? Or I may need a mediation server as a must requirement  to make lync voice work?

Thanks

WenFei