Can't summon multiple Phantoms in Anor Londo?

In Anor Londo, I can't seem to summon more than 1 friendly phantom at any given time. If I summon 1 phantom, all others disappear. Why can't I summon more than 1 phantom?

Solution 1:

PvP-related areas have a restricted amount of summon slots because of that area's covenant invaders take up said slots. You can lift that restriction using the Dried fingers as you noticed. Normally you can only have 4 players in a world (you + 3 others). Using the fingers adds 2 slots.

The Watchdogs of Farron take up slots in Road of Sacrifice and in Farron Keep. The Aldrich Faithful take up slots in Anor Londo.

Solution 2:

I'm not sure why, but this area seems to limit the amount of summons in this area. Possibly due to its simplicity, or due to its inclusion in Anri's storyline.

You can summon more phantoms if you use the Dried Finger, raising the limit to 2, however doing so can invite invaders more easily as well, so take care.