There is a prospective customer that is using "Virtual PBX".
They evidently are using KX UT113 phones, but I don't see them on the Vitual PBX website.

The customer evidently owns the phones and wants to get away from hosted (surprise suprise!. We are entertaining putting them on a Zultys, but they don't "support" this model.

My question is since I am completely unfamiliar with this phone I don't know how well it can be reconfigured and re-purposed. I know that some re-branded polycom, yeahlink,and Aastraphones, can't or are not easily converted back to the original manufacter settings.

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.