In a traditional landline telephone connection, a dial tone was a signal indicating that you were ready to make a call and that all of the systems were ready. Your phone was directly connected to, and relied on, the hardware and programming of a central system to be able to dial.
However, with a BluePrint hosted PBX phone system, the phones do not rely on a central or core system to perform some of their tasks such as dialing. When you go off-hook to dial, the phone is providing you a recognizable tone that is prompting you to start dialing. Dialed digits are then being held until it is determined dialing is complete. The phone then sends those digits to the main system to dial out.