The 5800-series telephones have been off the market for quite a while.

Try contacting VTech/AAT (Advanced American Telephones) and explain the situation. They may have a solution. It does seem odd to have a handset with an wrong number of digits in the ID.

https://telephones.att.com/att/index.cfm/product-support/contact-us/

If you check the existing handsets (the one that came with your telephone and the 1 expansion unit you already have) are the IDs 14 or 15 digits long?

VTech/ATT produced a 3358 telephone which looked very similar to the 5830/5840/5870 sets. While I've never seen or used one, the IDs may have been 15 digits long.


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