MyScript.kt

fun String.isPhoneNumber() = matches(Regex("\\d{11}"))

MyScriptSpec.groovy

import spock.lang.Specification

class MyScriptSpec extends Specification {
    def 'shoould check is string a phone number'() {
        expect:
            // '48123123123'.isPhoneNumber() // it does not work
            MyScriptKt.isPhoneNumber("48123123123")
            !MyScriptKt.isPhoneNumber("")
    } 
}