multiple additional prefixes may be reserved for test purposes. Many area codes reserved 999; 320 was also formerly reserved in Bell Canada territory. Other carrier-specific North American test numbers include 555-XXXX numbers (such as 555-0311 on Rogers Communications in Canada) or vertical service codes, such as *99 on Cablevision/Optimum Voice in the US. Telephone numbers Plant testing telephone numbers are carrier-specific, there is no comprehensive list of telephone numbers for ANAC services. In some communities, test numbers change relatively often. In others, a major incumbent carrier might assign a single number which provides test functions on its network across an entire numbering plan area, throughout an entire province or state, or system-wide. Some telecommunication carriers maintain toll-free numbers for ANAC facilities. Some national toll-free numbers provide automatic number identification (ANI) by speaking the telephone number of the caller, but these are not intended for use in identifying the customer's own phone number. They are used for the agent in a call center to confirm the telephone a customer is calling from, so that the customer's account information can be displayed as a "screen pop" for the next available customer service representative. See also Plant test number Ringback number References Telephone numbers Telephony signals