I would rule out calcium deficiency unless you have a good reason to think it is calcium related.
One thing I'd like to say about Phrags is that this issue seems fairly common when I visited some nurseries with good number of phrags. Almost all the phrags they had with very few exceptions, looked pretty much the same as shown in your photo or had their leaves heavily trimmed off. The commercial nurseries feed their plants on a regular basis. So, it can't be calcium deficiency and honestly, they don't look typical of calcium deficiency to me.
Anthracnose is a high possibility as I had one phrag that had this same symptom which stopped progressing further when I applied antifungal agent on the affected area. Fungus is everywhere and some plants especially under less-than-ideal conditions fall easy victim to fungal diseases.
Another phrag, my very first phrag, also showed similar issue, but it was due to dehydration. I was keeping the plant too dry. I switched the plant to wet feet condition as suggested and ta da! The plant just took off growing like a weed.
So, I would check first to see if you are watering your plant enough at all times. I saw Phrag Grande being grown in the bucket full of water. Some of these phrags really do like to have a lot of water always available at their roots.
Then, try spraying the antifungal and see if that stops any further damage on the leaves.