MessageBird created a separate parameter for voice messages.
When you send a message via the SMS API, just add 'voice=1' to your parameters. This works internationally for both mobile and landline numbers. With Text to Speech, the user always gets a phone call, whether to a landline or a mobile phone. If the receiver answers the phone, the sent message will be spoken by our Text to Speech computer. You will also receive delivery reports for Text to Speech messages.