AT command - USSD

AT+CUSD=1,"your_service_code",15

So for your case,

AT+CUSD=1,"*555*87*1234#",15

And don't forget the 15 at the end. This question helped me to solve the problem.

The format of the command is AT+CUSD=[<n>[,<str>[,<dcs>]]], and the second parameter, <str>, is a string that should be enclosed in double quotes. E.g.

AT+CUSD=1,"*555*87*1234#"
AT+CUSD=1,"1"

See 27.007 for more details about AT+CUSD and V.250 for general AT command handling.

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If your phone gives an OK response to AT+CUSD=1,1 that is really bad, it should have given ERROR. V.250 is crystal clear on this:

String constants shall be bounded at the beginning and end by the double-quote character