If you can't purchase the GNSS HAT, use an old Android phone and install Termux on it. Then run some bash scripts on the phone that will do the job.

The phone will communicate with your network over wifi, and replaces both the Raspberry PI and the HAT.

Can someone please explain to me the appeal of telegram (as opposed to signal)? Maybe I don’t understand something or am too old but how is it a safe haven compared to WhatsApp in the wake of the WhatsApp debecle. I just fail to understand.

Does anyone know if you could you this combo to creat some kind of VoIP gateway that could forward calls to an app on an iPhone over the Internet?

Back in the day we use to call such devices gateways.

Why isn't there something that turns a rooted Android phone into a VOIP <-> Cell network gateway? Something that allows incoming calls to be forwarded to SIP, and vice versa?

I haven't found something like that.

Nice I guess?

The hat itself is somewhat expensive (for only GSM), especially compares to LTE USB sticks which can do the same (and more). But of course not as compact and nice looking.

When would you use this SMS gateway versus something like Twilio, Plivo, or Bandwidth.com terminating a DID for SMS service?

I use gammu-smsd and forward it to iOS using Pushover.

For the serial AT modem I use random Huawei LTE USB stick that I bought for 15 EUR years ago.

It was so cheap because it was provider locked, but there are keygens floating around internet that allow unlocking it to work on any network.

I'm not sure I understand. Does this thing do a man in the middle. Do you have to configure your phone to use it?