Typical spec CAT5 gauge is 24awg, or no lower than 22awg to meet spec, it should have no problem carrying 600mA a couple meters using a single conductor. If it was a very short cable or cheap/generic they might've downsized the cable to 26awg.
The bigger issue that threw me at first when you talked about CAT5 was whether you were going to use RJ45 connectors, which can be a bit finicky about current so I simply wouldn't use any.
Bell wire, at least in the US is not a specific gauge but usually closer to 20awg. It wouldn't surprise me if recent years' copper prices have caused that to drop to 22awg and/or switch to copper plated aluminum.