Are PTR records added only by the ISPs that provide and manage the IPs?

I am administering a network with many locations and work with multiple providers I checked out with my DNS provider to change the PTR records for one of the IPs, they replied:

Dear Sir,

We can not modify the PTR records of the IP addresses that you provided, because these IP addresses are not managed by ZZZZZZZ.

You will have to contact the provider of the IP addresses to modify the PTR records.

Is this general practice and PTR records can be added only by the ISP companies that manage the IPs, or is just the practice for this DNS provider?

I think there's slightly more complexity than mdpc suggests. The letter is true: only the person to whom the relevant chunk of PTR space has been delegated can manage the PTR records in that space.

In the second paragraph they have made the assumption that the current delegate is the person who is responsible for the PA IP address space. That is usually a safe assumption, because the general structure of PTR records is such that they were delegated in chunks corresponding to the /24 IP address block they were in. But as mdpc points out, RFC 2317 introduced a mechanism for delegating subsections of those basic chunks. I have worked with organisations who have used that mechanism to get control of the PTRs for their sub-/24 netblocks. But it requires a highly clueful ISP, and a friendly and compliant one to boot; it is not, in my experience, commonly used.

You can walk the DNS tree yourself to see to whom your PTR space has been delegated, it is likely to be your current ISP, and not likely to be your current DNS provider, unless those happen to be the same person. You can then contact them to request either direct insertion of PTR records, or the delegation of your subsection to you or your representative.

This is not necessarily true and really depends on the ISP as to whether they want to delegate PTR record maintenance to you.

From what I recall, several years ago using one ISP based on an RFC I saw then on a moderately complicated procedure that was supported for just this thing. I do not know who widespread this RFC has been implemented or even if it was abandoned.

Using another ISP, any network range that was assigned to me I was allowed to maintain the reverse zone records myself.