Caller's buffer is too small.

This isn't a very common error anymore.

Most times this is caused by users playing around with the advanced settings of the modem.

If you don't remember what you changed to cause this error, uninstall and reinstall the modem to start.

Also make sure the speed isn't set to low, slighly higher is good, ie set to 38400, or 57600 is a good idea.

Under the port settings of your modem, make sure you didn't set the FIFO's to low, which is a common error among some techs.

You could also uninstall and reinstall dial up networking as well if you like.