It's a string of Hayes modem control codes.
AT - attention
H0 - pick up the phone, ending any previous call from the same modem
M0 - silence the modem speaker
V1 - set the modem to verbose mode, displaying English-language information instead of number codes
D - dial the following number
T - use touch-tone dialing instead of pulse dialing
8675309 - call JENNY-BBS
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