I’ll consider four general approaches:
- No additional server or services.
- Devices register to a VoIP provider.
- Devices register to a cloud PBX.
- Devices register to an on-site PBX.
For (1), you would configure both the HT and the phone apps to have fixed private IP addresses. This could be done with either static IP address assignments, or configuring your router to assign a static address to each of the three devices. You would then configure the SIP apps to send calls to the HT’s address, and the HT to send calls to one of the apps. Outbound should work fine, but incoming calls would be sent to only one app, because the HT has no way to ‘fork’ a call. This requires an app that can be set to not register (the HT is not a SIP server), but still ‘listen’ for incoming calls. Unfortunately, I don’t know which apps can do this. If you can’t find one that does this and otherwise meets your needs, you could modify one of the open source ones (Linphone and CSipSimple come to mind). I also don’t know whether there would be significant battery drain (the idle app must be listening but doesn’t normally receive any packets).
For (2), choose a provider that if you have no incoming numbers with them and don’t make any external outbound calls, monthly cost is zero. You’ll likely still have to make one payment e.g. £5. I’ll use Localphone as an example. Your smartphones would register to one SIP ID and the HT to a second (which you need to open a ticket to get). Incoming calls are straightforward and would ring both apps; the first to answer will get the call. On outgoing, you’ll call the second ID and the HT would use ‘2-stage dialing’ to accept a password and the destination number. You’d use the ‘number rewriting’ feature of the app to automate this.
With a PBX, your application is straightforward, but since you don’t want to use a Pi, here are some alternatives:
For cloud, some PBX providers, including 3CX and Vodia offer free trials, typically for one year. You can also set up FreePBX in the cloud. Choices include simple install-the-distro-and-it-runs providers such as Vultr (typically ~£5/mo.), Amazon or Google ‘free tier’ (some Linux skills needed and some limitations), or really cheap VPSs (see e.g. lowendstock.com) which cost almost nothing but may have poor voice quality or be unreliable.
For on-site, you might run a VM on a PC that’s always left on, run Asterisk on a router with open source firmware, or run FreePBX in a container on a system that is primarily doing something else (home automation, security cameras, file or media server, etc.)
