I used to be a front desk manager at a Hotel believe it or not

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The problem usually is that different switches take different amounts of time after you dial the 9 or 8 for the outside line to give a second dialtone. Dial the ISP's number from your voice phone first, and get a rough guesstimate for the amount of time it takes to get the outside line. If it seems to take longer than usual, put more commas between your 9 and your number, etc.
Xayd