If you want to test your Makerphone’s ability to place a call without using any minutes on your calling plan, here is a test number in the United States that repeats a test message several times but does not supervise, which means as far as your phone and calling plan are concerned, never answers:
+1 215-846-1212
While we’re on the topic, here is one that is ALWAYS busy (it’s been busy for at least the past 20 years):
+1 215-674-0096
If you’re calling from outside the USA, these calls are free only if your carrier doesn’t charge for unanswered calls or busy signals.
In the USA, only if your carrier is Ting, the following number, which plays a pre-recorded message three times, is always free:
+1 855-865-5971