At one time, Western Electric picked a model number for a phone, and the other companies which made similar models used the same number.
A #500 single-line rotary phone is basically the same whether it's made by Western Electric, Stromberg-Carlson, ITT or Northern Telecom.
Here are two six-button touchtone wall phones. The one on the left was made by Western Electric. The one on the right is from Stromberg-Carlson. Both are model 2851.
Confusion works in the other direction, too. The model number for a Western Electric two-line rotary-dial phone with mechanical hold is 515. ITT and Stromberg-Carlson use #575 for the same type of phone.