Problem with GBP telephone verification

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I am trying to verify a new GBP for our client by telephone. Problem is that the phone number listed in the GBP (which is automatically used for the verification call) is forwarded to the company's central phone. It is NOT (easily) possible to change that routing, as it is somehow hard-coded in the customer's phone system

I have started the call three times already but nothing happens. Could it be that Google stops the verification call when it realizes that the call is being forwarded?

Any idea on how to proceed? I am afraid that the GBP might get suspended or that telephone verification will no longer be available.