While receipt of a subpoena in the mail is in some circumstances valid if you reside in the state, it is not when you live outside the state, and thus outside the jurisdiction of Kansas Courts. One would think that Kline being a lawyer (albeit one who has had his license suspended in the past) and being the former "top cop" of the State he would be familiar with Chapter 22 of the Kansas statutes.
Since Kline now lives in Virgina, unless he wants to come back of his own free will and testify, there is much more that must be done to compel him to return.
Kline's statement that he will honor the subpoena means he's willingly coming back.
Anyone care to guess why Kline would agree to come back and testify for a domestic terrorist against Dr. Tiller?