ᐅ Connecting an IP Phone to Multiple Telephone Systems

Good morning,

this topic might be a bit too specialized for a house building forum, but there are quite a few experts here. As some of you might know from other threads, I have been working on setting up my network for a while. Now I have come across a somewhat tricky question:

I mostly work from my home office and currently use an IP phone provided by my employer, which is connected to their phone system. In the new house, I don’t want to have two phones on my desk (private and work), but just one that can handle calls on both (or multiple) numbers. The question is how to achieve this. From what I understand, the phone on my desk would need to be registered with two phone systems: my employer’s and my own. But is this even possible?

Additional information: I haven’t decided yet what I will use for my private phone system. Probably just a FritzBox, but I am also open to self-hosted or cloud-hosted phone systems (3CX?). Softphone solutions are only an option; I want to be able to pick up a handset and make calls even when the PC is off. I haven’t chosen a phone yet and am completely open to suggestions, as I suspect this will be the only factor I can influence.

This works without any issues. I have a Grandstream device here, and it is currently registered with three SIP accounts. One account is connected to the local Fritzbox, the second to the company (Sipgate), and the third is for the door station. By the way, Sipgate also works very well as a cloud phone system for private use. In terms of cost, you need to check whether it is worthwhile compared to a plan from your internet provider. It becomes especially interesting if you use mobile telephony for several family members through it.

I just checked, my GXV3370 could support up to 16 accounts.

Araknis schrieb:

No problem at all. I have a Grandstream here, and it’s currently registered with three SIP accounts. One account is connected to the local FritzBox, another to the company (Sipgate), and the third is for the door station.

With my employer’s Unify OpenScape system, this only works if all SIP accounts are registered with the same registrar. That probably won’t be the case, so the phone might not work. Which device are you using? Yealink seems to be quite popular these days?

Araknis schrieb:

By the way, Sipgate also works really well for private use as a cloud PBX. The pricing depends on whether it’s more cost-effective than an internet provider’s plan.

I haven’t fully understood this yet. My plan includes a landline flat rate with two phone numbers. So I guess I have one SIP account that I can register with. But what exactly is the PBX then? There has to be some system to manage the account, like storing a unified address book and so on, right? Am I missing something?

I thought of it like this: SIP account – PBX (FritzBox or 3CX or whatever) – endpoint device.
Although I can enter the SIP data directly into the endpoint devices, without a central PBX I wouldn’t be able to manage multiple devices (for example, for private home phone use), such as call transfer, shared address book, etc.

Araknis schrieb:

It gets more interesting if you also handle mobile telephony for several family members through it.

I don’t quite understand that sentence yet. Do you mean if I want to call the family members’ mobile phones? If that’s not included in my plan, I would install softphone apps on their mobiles and make calls over the internet, right? The softphones would be registered to the PBX as well, so I would have the central contacts there, correct?

squier23 schrieb:

But since I would like to have a central phonebook, that wouldn’t work with just plain DECT, would it?

Yes, it does—for example, with a Fritzbox and Fritzfons. The address book is stored on the Fritzbox and distributed to the devices via DECT.

squier23 schrieb:

I don’t quite understand that yet... My plan includes a landline flat rate and 2 phone numbers. So I have a SIP account I can register to. But what or who then is the telephone system? I need something to manage that account, right? To store unified address books and so on? Or have I completely misunderstood this?

For two numbers, you don’t need a telephone system if you just want to make calls. You need a PBX if you want features like call forwarding, call transfer, or similar. There are no unified address books in that case, as mentioned above.

squier23 schrieb:

I was thinking something like: SIP account – telephone system (FritzBox, 3CX, or whatever) – device.

You can assign multiple numbers from different accounts to a Fritzbox. However, I don’t know exactly how that looks on Fritzfons. I only have the different accounts set up on one phone. A shared address book should be possible, but transferring data between accounts would have to be tested.

squier23 schrieb:

I can also enter SIP data directly on devices, but then I don’t have a central system to manage multiple devices (e.g., for private home calls), allowing transfer/call handling, address books, etc.

Exactly, as described earlier.

squier23 schrieb:

I don’t understand this sentence yet. Do you mean if I want to call family members’ mobile devices? If that’s not included in the plan, I’d install softphone apps and call via the internet, correct? The softphones also register to the telephone system, so I would then have access to central contacts, right?

Sipgate works per user, not per device. You can also get SIM cards or eSIMs that are managed together—deciding which rings when, how calls are forwarded, etc. A shared address book via Sipgate, meaning in the cloud, is currently only available with Sipgate Team and Snom phones, as far as I know. In companies, this is usually handled via Active Directory or similar systems.

Araknis schrieb:

Thanks! That at least cleared up one issue. For my needs, I’ll probably just get a FritzBox and use FritzFons as handsets for my wife and kids.

However, the question remains how I can have just one desk phone in the office connected to two telephone systems (both my employer’s system and my FritzBox). I understand I need a phone that supports this. Your Grandstream is quite expensive but looks good. I would still need to check with my employer whether I’m allowed to connect my personal phone to their system, right? What information would I need for this? I assume it’s not simply SIP credentials but some kind of login data for their telephone system, basically something like CTI?

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