Where is telephone country code 919?

Country code +91 is India, and +91 9 is a mobile number. All telephone numbers beginning with +91 9 are mobiles; there are also both mobiles and landline area codes beginning with +91 7 and +91 8.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)