If they are running enterprise FW which is likely due to the PN that you posted, when the TFTP server is not reachable that is the expected behavior, you need to enter the activation code for registration or domain for MRA, both assume you use a Cisco call control in the backend.
There are plenty of threads that cover registration to 3rd party PBX while using enterprise FW, start by reviewing those. You'll have to provision all the configuration files the phone would expect from a Cisco call control in the same format, replacing your VOIP provider info as required. If the thread refers to 3PCC/MPP devices, that is not applicable to you
You can pay to migrate to the 3PCC/MPP FW that would enable you to use a built-in web server for all the configuration you would need with your provider, all details at upgrade.cisco.com
HTH
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