You have a 28.8 modem, but you always connect at 26400 or lower.
There are many reasons why people do not often connect at their modem's optimal speed. This also applies if you have a 33.6 modem and connect at 31200 or lower.
The most common reason for a low-speed connection in telephone line noise. 28.8 and 33.6 modems push the telephone network to its limits; to connect at your top speed, telephone conditions must be ideal. You must have a good line; that is, there must not be any noise, fuzz, or distortion on your telephone connection. This is an unavoidable consequence of the way modems work.
If you have an external modem on a PC, you should be aware of your serial port's speed.
You may also have a software problem. Often times, if you are running both fax software and internet software, there will be a conflict. Fax machines run at 9600 or 14400. If your fax software initializes your modem and does not reset it, your modem will only speak at 9600 or 14400. Contact the manufacturer of your fax software.
You may have a bad modem init string. Modem init strings are found in many places - Windows 95's Dial-Up Networking and Modem Control Panels, Windows 3.1's winsock, MacOS's PPP ConfigPPP and FreePPP images_files. and within many software programs. Contact your modem's manufacturer or the documentation that came with your modem for an appropriate initialization string.