<h:body>
    <p:messages autoUpdate="true"/>
    <h:form id="mainform">
        <p:commandButton id="button1"
                         value="Configuração">
            <p:ajax listener="#{config.carregaSbcDevices}" update="panelAU"/>
        </p:commandButton>
        <p:outputPanel autoUpdate="true" id="panelAU">
            <p:selectOneMenu value="#{config.configSCBEsc}">
                <f:selectItem itemLabel="Selecione..." itemValue="" />
                <f:selectItems value="#{config.devicesName}" />
            </p:selectOneMenu>
        </p:outputPanel>
    </h:form>
</h:body>

public void carregaSbcDevices() {
    if(this.devices == null) this.devices = new ArrayList<>();
    this.devices.clear();

    if(this.devicesName == null) this.devicesName = new ArrayList<>();
    this.devicesName.clear();

    session.getDevices(this.devices);
    for(SBCDevices device : this.devices) {
        this.devicesName.add(device.getTargetName());
    }

    System.out.println("Numero: " + this.devicesName.size());
}