Thanks, I will look into those.
Is there a reason you don’t want to just use a SIP service by the way? It would certainly be cheaper than that board plus a mobile plan.
At the moment I am using a SIP service - at least I think I am. Is JMP.chat a SIP service? It works to a degree for me but there are many drawbacks. I can't get a number from my country, many services recognize and block the phone number as a VoIP phone (for example, telegram), and I have to trust an additional intermediary to get my unencrypted SMS and 2FA codes. I do trust the service providers to a reasonably high degree, but I don't think it is wise to associate their numbers with sensitive accounts such as my bank account.
The VoIP idea is not the reason why I began playing with these boards in the first place, though. It is an idea that I thought of while playing with these modules. And I don't use mobile plans. I use sim cards that I can top up when needed, so the cost is about $5 every 6 months or so for the home base, and about ~$5 a month for the data I use for my mobile router. The reason why I want to control the device through the raspberry pi is that then I also have more fine-grained control over the top-up process and I "own" my phone number without an additional intermediary.
The Netherlands. I just checked and it is €10 for 5GB/month or €8 for 5GB month if one pays for a year: https://www.lycamobile.nl/en/bundle/monthly-plan-xs/?m=12
But in the shops they have signs with deals and you can get often get codes for cheaper. They print out the codes for you and you can top-up by issuing a USSD code when needed. I buy a stack of codes, type them into my notes app, and use them when I need them. Since I commonly have WiFi available I sometimes go months without activating them.
In the home base I wouldn't need data, as it is interfacing through the raspberry pi. Most of my phone activity is getting codes for 2FA for some important services that force me to. Since I rarely make calls, I can top up €5 of credit when needed. I need to make sure to top up every 6 months so that the SIM does not get deactivated. That's why estimate ~€10 for every 6 months.
This works for me because I do not use the standard phone features (SMS and calling) often. If it were up to me, I would not have the phone functionality at all and simply keep the data. But sometimes I need a phone number. The purpose of this project is to keep the convenience of having a phone in the rare cases I need it without constantly having on me a device broadcasting unique identifiers. As for why go self-host route... I think the same can be said for many other self-hosting projects, it is nice to have more control. It is not really about the money. Saving money is part of the fun, and that is an easy way to "justify" what I'm doing... but the reality is that any savings argument probably gets thrown into the BS pile when we account for the time invested, failed attempts, and the extra stuff I buy for trial and error. Ultimately for me it's really not about money, it's about being mindful, learning, and having fun.
I have found VoIP numbers from Canada and the US, I am not sure if it is easy to get one from the Netherlands. I have experience being banned from sites immediately after registering with the VoIP number because of "suspicious activity".