Click the send TCPA Invite button from the Contacts table.
Then select Send to Imported File and click the Choose File button.
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It will show you how many contacts on the list have valid mobile Mobile Numbers. The sample below shows 2 imports of contacts that have not yet been invited. The 1st file has 4 mobile numbers and the 2nd file has the same 4 mobile numbers plus 2 landlines. Only the Mobile Numbers show up as valid.