Hello, Yes this happens when you add a additional handset to the system when all the intercom numbers are full. When registering a new handset, it "kicks out" on of the other handsets when it registers. The only way to correct that problem is to default your base and register each handset again, one by one. You won't lose your handset settings, but you will lose your messages and greetings, so check them before doing so. To default your unit, get all your handsets and go to your base. Unplug the power from your base. Hold down the page/registration key on the bottom of the base and keep it held down while you apply power to the base. After applying power, keeping the button down, count to 20. Then release the button and get each handset. Register each handset, and they will number themselves 1 through 4. If you need additional assistance, service or need to purchase another handset or base (you can register each handset to up to four bases to increase your range, this is unique feature in the cordless industry), you can go to www.cordlessrepair.com
for all your Siemens cordless needs. Good Luck ! Let us know how it goes, we have a testimonial page on the website for people to leave comments on the technical tips or assistance they receive from our technicians.