If you can use those sims in mobile phones then I’d call them mobile plans. Can I ask what country you are in? Here in the US, mobile service costs a lot more. I have been getting SIP service from vitelity.net but twilio.com and voip.ms are better known here. I don’t know about jmp.chat.
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 haven’t had trouble using a hosted number for banking and it feels better to me than using a mobile number. The cheap mobile providers (MVNO’s) I use here are sketchy, mobile numbers change all the time, etc. I use a VoIP number as my permanent stable number and forward it to my mobile. So if I switch mobiles, I just change the forwarding. In theory you can port phone numbers between carriers but I’ve had significant hassle doing that. That’s just here though. It may be different where you are.
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”.