I wonder what the lowest Birkenhead telephone number was? Perhaps they started at 100!
I don't know about Birkenhead itself but I do know about Upton, in 1896 there were only 8 telephones in Upton, and they had numbers 2401 to 2408 (no exchange).
By 1911 Upton was on the Birkenhead exchange and the lowest number in Upton was Birkenhead 551.
By 1920 Upton had its own exchange, and the lowest number was Upton 1, when the first automatic exchange was installed, the lowest number became UPTon 0001. In 1952 the exchange name changed to Arrowebrook, making the lowest number ARRowebrook 0001.