Aside from a potential cost savings for 3rd party sip phones (which may or may not be the case) what are the pro's and con's of using 3rd party SIP phones with UCM6.1?

Also, can a 2811 or 2851 be configured as a pure sip voice gateway? I.e can we configure the router to be a sip gateway on the UCM6.1?

If we have a sip provider for PSTN, and can and do run gateway as sip only, do we still need to purchase ip2ip gw license?

thank you