65535的移动网络代码对于cttelephonynetworkinfo().subscriberCellularProvider意味着什么?.mobilenetworkcode?

我在看别人写的一些没有文档的代码,为什么这段代码要和65535做比较?

class func canMakePhoneCall() -> Bool
{
    guard let URL = URL(string: "tel://") else {
        return false
    }

    let canOpenURL = UIApplication.shared.canOpenURL(URL)
    if canOpenURL == false
    {
        return false
    }

    let mobileNetworkCode = CTTelephonyNetworkInfo().subscriberCellularProvider?.mobileNetworkCode

    let isInvalidNetworkCode = mobileNetworkCode == nil
        || mobileNetworkCode?.characters.count == 0
        || mobileNetworkCode == "65535"

    return isInvalidNetworkCode == false
}