Welcome to the community!
Sorry to hear you've incurred an unexpected charge. Had you called the taxi company directly or did you use #TAXI?
If you called the company directly, the length of the call would have been included in your Unlimited minutes. However, #TAXI is a third-party service and there is a charge is for using that service. You can view more information (and pricing) regarding #TAXI here. As mentioned in their pricing information, airtime charges also apply. It's likely you did not incur airtime charges because you have the Unlimited minutes.
It should also be noted that there may be additional fees for other quick access numbers (see here).
Hope this helps 😀