Kotlin Coroutine wait for Retrofit Response

I'm trying to use the Android MVVM pattern with a repository class and Retrofit for network calls. I have the common problem that I can't get the coroutine to wait for the network response to return.

This method is in my ViewModel class:

private fun loadConfigModel() {
    val model = runBlocking {
        withContext(Dispatchers.IO) {
            configModelRepository.getConfigFile()
        }
    }
    configModel.value = model
}

In ConfigModelRepository, I have this:

suspend fun getConfigFile(): ConfigModel {
    val configString = prefs.getString(
        ConfigViewModel.CONFIG_SHARED_PREF_KEY, "") ?: ""

    return if (configString.isEmpty() || isCacheExpired()) {
        runBlocking { fetchConfig() }
    } else {
        postFromLocalCache(configString)
    }
}

private suspend fun fetchConfig(): ConfigModel {
    return suspendCoroutine { cont ->
         dataService
            .config()  // <-- LAST LINE CALLED
            .enqueue(object : Callback<ConfigModel> {
                 override fun onResponse(call: Call<ConfigModel>, response: Response<ConfigModel>) {
                    if (response.isSuccessful) {
                        response.body()?.let {
                            saveConfigResponseInSharedPreferences(it)
                            cont.resume(it)
                        }
                    } else {
                        cont.resume(ConfigModel(listOf(), listOf()))
                    }
                }

                override fun onFailure(call: Call<ConfigModel>, t: Throwable) {
                    Timber.e(t, "config fetch failed")
                    cont.resume(ConfigModel(listOf(), listOf()))
                }
            })
    }
}

My code runs as far as dataService.config(). It never enters onResponse or onFailure. The network call goes and and returns properly (I can see this using Charles), but the coroutine doesn't seem to be listening for the callback.

So, my question is the usual one. How can I get the coroutines to block such that they wait for this callback from Retrofit? Thanks.