There are many betamax voip services such as voipgain, voipcheap, voipstunt which ask you to supply a phone number and a destination phone number when making a call (as can be seen in the free trial window on their webpages-I believe them to belong to betamax). Do they allow a user to select a phone number arbitrarily for spoofing?
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These trunk providers can indeed send anything to the telephone network, but whether they allow you to select the number is based on their own policy.
But in the case you asked, where you provide your phone number and the destination number, they have to actually ring your phone first before they connect the destination, so this is kind of authentication.
Also, good providers also requires similar authentications if you require your number (instead of the provider's) to be used as caller id.