More than half of the readers of OOTS do not play D&D at all, and half of those that do play only casually. The average reader does not know what "calling" means in the context of D&D, especially when the plot makes frequent use of the spell sending, which is a poorly-named method of "calling" another person up and talking to them, as you would on the phone. Thus, I don't use the word. I use "summoning" regardless of whether it was a summoning or calling spell, because it doesn't really matter.
As was pointed out, I don't always follow the rules...including the rules on what things are named.
Summoning/calling spells work if cast within the Cloister. As pointed out by Zevox, any extradimensional travel spell cast from within the Cloister works normally just by the nature of the effect. Yes, that makes Celia's mention of Dorukan's specific exception somewhat redundant, but A.) that was a fairly complex bit of exposition and I wanted it to be certain that everyone understood that summoning could still happen, and B.) it provided me with a good punchline and a way to show that Lirian and Dorukan were cheating on their oaths. So I tolerated some redundancy.
Also, as a general rule of thumb, no one should say the sentence, "There's no (or no other) possible narrative purpose for Rich to have done X!" until the story is completed. Because there's always a narrative purpose, you just haven't thought of what it is.
Sometimes, the narrative purpose is to set up something that won't happen for 200 strips. Sometimes, it's to enable a joke, or to make the narrative easier to understand for non-players. Sometimes, it's just to stop people from wondering why they didn't use a different spell. And sometimes, it's just a deliberate red herring because this is a serialized story that won't reach its ending for another few years and I don't want everyone to predict what's going to happen along the way. All of those (and more) are legitimate reasons to have a character say or do something, even if not all of them apply to your personal reading experience.