When you try connecting to the internet using a MAC 10.3.9 and above the dial-up shows as connecting and after sometime shows as disconnecting.

Most of the USB modems available in the market are only compactable from 10.3.9 onwards. Please read the compatibility chart on the box before purchase. If unsure of the version of MAC you are using click on "Apple" icon and select "About this MAC"

If you are using a software for your USB modem make sure that the settings are correct.(for e.g. access point, dialup number etc) make note that most internet service providers do not provide username and password with USB modems. if you have entered any username or password, delete it.

Mac 10.5 and above

If you are using MAC 10.5 and above you can still connect without the software.

Plug the USB modem in and click on the "apple" icon and click on "system preferences". here find an icon called "Network" and open it. You should get a pop-up with a message "New port detected". Click on "OK". Now on the left of your screen you have all your connections showing for e.g. Airport, Bluetooth, VPN etc. Highlight your USB connection from the list and click on "Advance" at the bottom of the screen, it will open another small window with a tabs like modems, TCP/IP etc. Click on the "modem" tab. Here select the "Vendor" as "generic", "Model" as "GPRS/3g". it will show a new box with "Access point" (APN). Type the access point name of you internet service provider. If not sure of the access point name then please contact your internet service provider. Now click on "Ok". It will take you back to the old screen. Now select the configuration as default. The telephone number as *99# . Leave the username and password blank unless specified by your internet service provider. Click on "apply" and you are ready to connect to the internet.

Mac 10.3.9 and 10.4.X

On a Mac 10.3.9 and 10.4.X, where "X" is a number. The software is required to connect to the internet as here there is no option to enter the access point. So you will have to install the software you got along with modem. Most USB modem or wireless USB modems have the software inside the modem so there may be no CD provided to you. Different software will have different ways of navigation. some may have options like "Tools" or some may have "Settings". Here we need to make sure that the correct profile is selected with the correct access point. For some service providers access point name may differ from Prepaid and Postpaid/Plan. make sure you have the correct access point according to your plan you have with your service provider. Once the software is taken care of, follow the below steps.

For MAC 10.3.9 and 10.4.X  click "Apple" icon and click "system preferences". Open "Network". Here select the location as "Automatic". from the "show" dropdown select your USB modem from the list. Under the PPP tab where it says service provider, enter a name (dose not matter what we type in here). Leave the username and password blank unless specified by the service provider. Enter the telephone number as *99# . Now click on the "Modem" tab and where it says modem from the dropdown select the modem of your service provider. leave the rest of the settings the same unless you would want to change it as it is optional. Click on the "Apply Now" button at the bottom of the network window. Once the "Apply Now" button fades out close the network window and open the software and click connect. 

Note: If after all these steps if you are still not able to connect to the internet then contact your internet service provider and raise a trouble ticket or follow the instructions of your internet service provider.