The Modem has a DN assigned to it, so there are a couple ways.
First of all, You need to enable the modem. This can be done though Element Manager (Resources Dial-Up Interfaces -Modem Dial-In Parameters) , or Feature 9*8
To find out the DN of the modem, use Feature 9*0
You can either assign a line to the DN (so the modem will answer incoming calls); have a person transfer you to the Modem exension; or if you have an AA, you can use a dial string (with pauses) when setting up a connection on your computer (such as 4165556666,,,,2253) That will make your computer dial 419-555-666, wait a few seconds, then dial 2253.
What are you planning on using the modem for? remote management though element manger does work, but it's PAINFULLY slow. We use it for a remote employee who needs to dial-in for network access while on the road.