What is the ruling on a volunteer collecting mobile numbers of those wishing to subscribe to the Da'wah Office's mobile service, and then exploiting these numbers to send them promotional messages for his own financial gain?

If a person collects numbers under the umbrella of a Dawah office and then uses them for his personal gain, he has betrayed the trust and misused the numbers, and he must desist from that. The owners of the numbers authorized him to send Dawah messages only, and did not authorize him to use them for other purposes.