My ques. is: What can be done to maximize the range... (by 5 users)

Obtain an amateur radio license with appropriate qualifications which permit you to modify radio equipment for use by licensed amateurs.

Then (If the 902 - 928 MHz band is legal for amateurs in your jurisdiction),

You may be able to add an RF amplifier to the base and one to the handset.

In Canada, amateurs have secondary status in this band and may not interfere with those with primary status (check with Industry Canada)

In the US - check with the FCC and ARRL for info
before operating on this band in your location.

Place the base at the highest point of your house possible.
Keep it away from the fridge, and washer and dryer along with fans and air conditioners if this is possible.
Also if you work outdoors or go on the patio etc etc place it near a window.

By removing as many obstacles as possible and placing the base and/or antenna as high as possible will enhance the range. Maintain as much line-of-sight transmission as possible.