Cell phone numbers

 Most cell numbers lead to a busy signal, but occasionally an answering machine or someone will pick up. To enter a cell number, go to "Contacts", and press A. If you enter phone number cheats from Grand Theft Auto 4, you will hear a unique tone, including a dog whistle tone.
The following phone numbers lead to an answering machine:
389-555-0133
389-555-0144
369-555-0150
389-555-0160
389-555-0188
389-555-0199
The following phone number has a dial-up sound:
389-555-0155
The following phone numbers have a "placed on hold" song:
389-555-0139
389-555-0166
The following phone numbers have someone pick up and say "Stop prank calling me!":
389-555-0122
389-555-0159
389-555-0177
The following phone number has someone pick up and hang up:
389-555-0129
It seems most numbers entered as 389-555-01xx (replace "xx" with any digits) will get some kind of response.