My understanding is that you have a prospective customer who has been using KX-UT113 on a hosted service but now wants to back-off the hosted stuff. The customer wants to continue using his/her KX-UT113 hardware which does not work with the system - Zultys that you would like to deploy. As someone who has integrated several phone systems,i would if the Zultys has analog extensions use a grandstream GXW-4108. I suppose KX-UT113 work with GXW4208. Connect some analogs to the line puts of the GXW-4108 and have KX-UT113 register to this. It should work great.