According the Express Fax/Modem User's Guide the ":" command is the same as
the "W" modifier, except that the ":" can detect most calling card tones.
The "W" modifier is used to make the modem detect a dial tone before
sending the next digit, for example, calling card number. In some
instances, the modem will not be able to recognize the ":" command using
the Express Modem software version 1.1.2 that comes with the GeoPort
Telecom Adapter. The solution is to use commas to delay the modem to wait
until the long distance carrier dial tone is sent. The comma will delay
the modem 2 seconds for each comma.