dnd-5eplanesspellssummoning
If you secure an area with the Mordenkainen's private sanctum spell and choose the effect:
Planar travel is blocked within the warded area.
Does this prevent summoning creatures into the area via spells like infernal calling (XGtE, p. 158)?
Planar travel, in this instance, could either mean "willingly moving between planes" or "moving between planes under any circumstance", and I don't know which is meant by the spell.
You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the Duration, creatures can't Teleport into the area or use portals, such as those created by the gate spell, to enter the area. The spell proofs the area against Planar Travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the Plane Shift spell.
Spell description doesn't specify the figure of the warded area, so we can assume we can shape it as we want. Red squares are (your teleportation circle area, 5x5x10 area) excluded from warded zone. The only way enemies can teleport in is to know the exact location of teleportation circle within the fortress. You can ward you fortress as following:
DM Purview
There are no official 5th edition rules about these kinds of specific situations. A DM is free to decide.
On the one hand, you could argue that an area warded against planar travel is only warded against physical (and possibly astral) travelling. And when a devil (or, indeed, anyone else) dies in such an area there is no magic in the world that could possibly stop the soul or essence from departing to the afterlife.
On the other hand, a DM may decide that in his campaign this is precisely a way - possibly the only way - that a devil can be disposed of permanently. (If it stops the souls of other creatures from entering the afterlife as well it could even raise some awkward questions - maybe this is how powerful ghosts and wraiths are made!). NB: This is slightly against the monster manual text regarding such things, but 'fluff text' is always free to be adjusted/home-brewed by a DM.
Best Answer
The text says "Planar travel is blocked" with no other exceptions or restrictions, which means that both of your specified scenarios would be blocked, regardless of how or why it is happening. Therefore, as long as the summoning spell is summoning a creature from another plane, it would be blocked.
For example, Instant Summons would be blocked if it summoned the object from another plane, and most of the Conjure X spells would be blocked as they pull creatures from another plane.