I doubt anyone would pay that close attention during production as to actually set the dial to a particular number.
It would be interesting to know though.
Although I do like your theory as to what the "in-universe" dial is for and how it is set. Could be that only when you place a "call" do you need to set the dial for the Farnsworth you are trying to reach. If you are receiving a call, might not matter where the dial is set.
I assume each unit's number is "hard-wired" so as you say the dial would set for outgoing calls.
There are other warehouses as well, so maybe the Farnsworths are divided among them.
No there is only ONE warehouse in the world at any time and is placed where the power in the world is seated at that time.
Warehouse 1 was in the Kingfom of Macedonia 356-323BC.
Warehouse 2 was in Library of Alexandria.(egypt) 323-30BC.
Warehouse 3 was in the Western Roman Empire 30BC-434.
Warehouse 4 was in the Hunnic Empire 434-453.
Warehouse 5 was in the Byzantine Empire 453-813.
Warehouse 6 was in the Khmer Empire 813-1219.
Warehouse 7 was in the Mongol Empire 1219-1260.
Warehouse 8 was in the Holy Roman Empire 1260-1517.
Warehouse 9 was in the Ottoman Empire (Turkey) 1571-1566.
Warehouse 10 was in the Muhgal Empire (India) 1566-1725.
Warehouse 11 was in the Russian Empire (Moscow) 1725-1830.
Warehouse 12 was in the British Empire (London) 1830-1914.
They decided to break with tradition and put Warehouse 13 in an out of the way place. South Dakota 1914-present.
That's why Artie had to give Claudia Philo's, there aren't any other available to W13. Although they do have the blueprints, they could always build another. Although, if there are only a limited number of frequencies, the blueprints may only to build new ones to replace destroyed units and to repair broken ones.
Artie gave Claudia this special one to show how much he trusts her and cares for her. I'm sure they must get damaged or break down but Artie has already said that they don't understand how they get their power and exactly how they work. It is also possible that just those four exist but it seems unlikely. Who knows what else is "kicking" around the Farnsworth Aisle.