Twilio API - How to send concurrent requests

I need to parallelize our calls to send text messages so that we can send more than one at a time.

We have an existing long standing Twilio Account. When I try to parallel the call to send a text I get a "Too Many Requests" exception from Twilio.Exceptions.ApiExceptions (429) https://www.twilio.com/docs/api/errors/20429.

This is our method that calls Twilio to send the text:

    public async Task<MessageResource> SendMessage(string body, string ToNumber)
    { 
        Twilio.TwilioClient.Init(config.apiKey, config.apiSecret, config.accountSID);

        var message = await MessageResource.CreateAsync(
            body: body,
            from: config.fromNumber,
            to: new Twilio.Types.PhoneNumber(ToNumber)
         );

        return message;
    }

This is the line from which I'm trying to invoke the method that calls the method above.

   reminders.ToList().AsParallel().WithDegreeOfParallelism(2).ForAll(async r => { await ProcessReminders(r); });

Surely the API supports at least 2 concurrent requests? This is a similar question but its about a year old and I'm not sure it applies as this is a longstanding account. Twilio API - Too Many Requests when sending SMS

As of version 5 I believe twilio made the methods for creating the clients and sending messages went from instance to static. Is there some setting or new means of sending messages concurrently?