Hi Olga,
From my point of view PI is no the best tool to call a REST service, as pointed out you can do it directly via SAP Netweaver Gateway (a free Netweaver addon, integrated if you have SAP NW 7.4 in your ECC).
There are third party adapters or the AXIS adapter to consume REST services, we can think that the PI must be the central integration, but are we obfuscated to use always SAP PI as PI developers?. If the endpoint is inside the DMZ in your client organization, from my point of view using PI is like using a sledgehammer to crack a nut (in Spanish we say to kill a fly with a cannon shot, funniest 
) because SAP has yet a tool thought and ready for Restful architecture and we don't should forget that PI is a SOA Architecture, also you can even install Gateway in the ABAP stuck in PI. However, sometimes depending of your client and business consideration you can take PI AXIS option.
Hope this helps.
Regards.