Connect IP phone with multiple telephone systems

But the question still remains how I can have only one landline phone in the office and use it on two telephone systems (i.e. the employer's and my FritzBox). I need one that allows this, as far as I understand.

Multiple accounts on one phone are really not unusual, just google IP phones, find one that fits the price, and check the technical specifications.

I would now have to clarify with my employer whether I can/may connect my private phone to their system, right? What data would I need for that? Those aren’t directly the SIP data but some login data to their telephone system, so basically via CTI?

Just google how to set up internet numbers in FRITZ!Box, it’s described there. Just ask your employer, there certainly must be access data. He just has to hand it over.

Don't you have a softphone on the company computer?

Yes, we do, but I want to have something in my hand and above all be able to make calls even when the computer is turned off.

Then there are also login credentials... what software is that?

Then there are also access data... which software is that?

Currently estos / ProCall, but they probably want to switch to something else. However, I never entered the access data myself; the computer comes preconfigured and without any admin rights. But with the phone, they were smart enough not to change the default password for the admin web interface, maybe I can find something there.

If the employer doesn't provide any data, maybe try using call forwarding.

Otherwise, I solved everything with the Fritzbox.
With o2 I booked 3rd phone numbers and with sipgate an additional one because calling to country zone 2 was cheaper, but by now almost only WhatsApp calls are used.