It really depends on how the reverse was delegated to you. ISP's that subnets /28 or /29 blocks usually require your to create a special zone so that they can delegate a block of addresses to you. For single hosts, proper delagation is really a PITA.
Some ISP use a special CNAME trick - they setup a cname for the reverse to a forward zone that you control. That specific hostname that they cname to is then setup with a PTR record in your zone.
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