"Eight" does not work on a landline phone?

Question

Telephony

Hello. The problem is this: I bought a new landline phone (Gigaset DA 710) to replace the old one, connected it, checked its operation, and left. Then they contacted me from the same department with a question? Why doesn't long-distance calls (starting with 8) work for us on other phones?
As a result, it turns out that when a new device is connected, calls from the eight work on it, while the rest fail. When you turn off the phone, everything falls into place.
What is the problem?