Connect IP phone with multiple telephone systems

Good morning,

this topic is probably a bit too specific for a home construction forum, but there are quite a few experts here. As might be known from other threads, I have been planning the network topic for some time. Now I have again come across a somewhat stubborn question:

I mostly work from home and currently have an IP phone from my employer, which is registered with their telephone system. In the new house, I don't want to have two phones on the desk (private and work), but only one phone through which I can be reached and make calls via both (or multiple) numbers. Now the question arises how I can accomplish this. As I understand it, the phone on the desk would then have to be registered to two telephone systems, namely my employer's and my own. But is that even possible?

Further information: I do not yet know what I will use as a private telephone system. Probably just a FritzBox, but I am also open to self-hosted or cloud-hosted telephone systems (3CX?). Softphone telephony should only be an option; I want to be able to pick up a handset and make calls even when the PC is off. I have not yet decided on a phone, I am completely open, I assume this will be the only factor I can influence?

Works without any problems. I have a Grandstream here, and it is currently registered with three SIP accounts. One account runs on the local Fritzbox, the next on the company (Sipgate), and the third is the door station. By the way, Sipgate also works great for private use as a cloud phone system. In terms of price, you have to see if it is worthwhile compared to an internet provider's tariff. If you also do mobile telephony with several family members through it, it slowly becomes interesting.

Just checked, my GXV3370 could support up to 16 accounts.

No problem. I have a Grandstream here and it is currently registered with three SIP accounts. One account runs on the local Fritzbox, the next on the company (Sipgate), and the third is the door station.

With my employer's unify openScape, that probably only works if all SIP accounts run on the same registrar. That probably isn’t the case here, so the phone is already out of the question. Which device do you use? Yealink seems to be kind of popular?

By the way, Sipgate also works great for private use as a cloud PBX. In terms of price, you have to see if it’s worth it compared to an internet provider’s tariff.

I haven’t quite understood that yet.. my tariff includes a landline flat rate and 2 phone numbers. That means I have a SIP account that I can register with. But then who or what is the phone system? I have to manage the account via something, like storing unified address books and such? Am I understanding something completely wrong?

I was thinking something like this: SIP account - phone system (FritzBox or 3CX or whatever) - endpoint device.
Whereas with endpoint devices I can also enter the SIP data directly but then I don’t have a central system through which I could manage multiple endpoints (e.g., for private telephony at home), i.e., call transfer/consulting, address book, etc.

It gets interesting if you also do mobile telephony with several family members through it.

I don’t quite understand that sentence yet. You mean if I want to call the family members’ mobile devices? If that’s not included in the tariff, I would install softphone apps there and then call via the internet, right? The softphones are also registered at the phone system and through that I also have the central contacts, correct?

But since I would like to have a central phone book, that wouldn't work with pure DECT anyway, right?

Yes, for example with the Fritzbox and Fritzfons. The address book is then on the Fritzbox and distributed to the devices via DECT.

I somehow don't quite understand that yet.. I have a landline flat rate included in my plan and 2 phone numbers. That means I have a SIP account I can register with. But who or what is then the telephone system? I have to manage the account somehow, like storing unified address books and such? Did I completely misunderstand something?

You don't need a telephone system for two numbers if you just want to make calls. You need the system if you want to forward, switch, or do something like that. Unified address books then of course don't exist, see above.

I thought basically like this: SIP account - telephone system (FritzBox or 3CX or whatever) - device.

You can assign several numbers from different accounts to the Fritzbox. How that looks on the Fritzfons, I don't know. I only have the different accounts on one phone. A shared address book should be possible then; whether transmission between accounts works, you would have to try.

Whereas I can also enter the SIP data directly on the devices, but then I have no central system to manage multiple devices (e.g. for private telephony at home), i.e. transfer/switching, address book, etc.

Exactly, as described before.

I don't understand that sentence yet. Do you mean if I want to call the mobile devices of family members? If that's not included in the plan, I would install softphone apps there and then call via internet, right? The softphones are also registered at the telephone system and then I also have the central contacts via that, correct?

Sipgate works per user, not per device. But you can also book SIM cards or eSIMs that can all be managed together (what rings when, what is forwarded how, etc.). A shared address book via Sipgate, i.e. in the cloud, as far as I know currently only works with Sipgate Team and Snom phones. In companies, this is usually done via AD or similar.

Thanks! That at least solved a knot. For my needs, I will probably just get a FritzBox and Fritzfones as handsets for my wife and children.

But the question still remains how I can have only one landline phone in the office and use it with two telephone systems (i.e. my employer's and my FritzBox). I need one that allows this, as far as I understand. Your Grandstream is relatively expensive but looks good. I would now have to clarify with my employer whether I am allowed to connect my private phone to their system, correct? What data would I need for that? Those are not directly the SIP data, but some kind of login information to their telephone system, so basically via CTI?

Don’t you have a softphone on the company computer at the office?