Extend phone info

diff --git a/PERSONAL_INFRA.md b/PERSONAL_INFRA.md
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@@ -191,7 +191,40 @@ https://github.com/alexpdp7/gemini_blog
 
 ## Phones
 
-I wanted to eliminate my landlines, because I get a ton of spam there. However, I need to provide calls between my home and another home using physical phones, so I set up FreeSWITCH and two GrandStream HT801 devices.
+I wanted to eliminate my landlines, because I get a ton of spam there.
+However, I need to provide calls between my home and another home using physical phones (people like wireless headsets- smartphones are not really well designed for extended phone calls).
+
+The key to this is the SIP protocol.
+You can get classical phones that work using the SIP protocol, or ATA devices that turn a regular phone into a SIP phone.
+
+I installed FreeSWITCH from the [OKay repo](https://okay.network/blog-news/rpm-repositories-for-centos-6-and-7.html).
+FreeSWITCH comes with a fairly complete default configuration.
+By default it will set up extensions in the 1000...1020 range, with a configurable single password for all extensions, plus some extensions for test calls, etc.
+
+The major difficulty in setting a SIP server is networking.
+I run FreeSWITCH in an LXC container on Proxmox.
+I expose the SIP server's SSL TCP port to the Internet, plus a range of UDP ports, using iptables.
+(I consulted some SIP forums, and apparently there are no major hardening requirements in exposing a SIP server to the Internet, although I think maybe it's better to use a SIP proxy.)
+You can also use STUN/TURN servers, but I had lots of trouble getting that set up.
+Also by default, FreeSWITCH figures out a public IP- if you want to get FreeSWITCH working behind a VPN, you need to disable that.
+
+For the phones, I bought and set up two Grandstream HT801 ATA devices.
+Those are quite cheap (around 40€), but they are quite fancy professional network devices, with a rough but featureful UI (they can do OpenVPN, SNMP, etc.).
+They connect directly to FreeSWITCH over the Internet, autoconfiguring via DHCP, so in theory they could work anywhere in the world with a network connection.
+After configuration and assigning an extension, you only need to connect cheap wireless phones to them, and start making calls with the 1000...1020 extensions.
+
+For testing and occasional calls I use [Baresip](https://github.com/baresip/baresip) from F-Droid in my smartphone, and from Debian in my laptop.
+For smartphones, SIP has the drawback that it requires a persistent connection to the SIP server to receive calls- thus draining the battery a bit.
+Some SIP setups use push notifications to get around that, but that seemed to be complex.
+So the only devices that are connected 24/7 are the ATAs, I use my smartphone and my laptop occasionally.
+
+SIP allows many other interesting stuff such as:
+
+* Instant messaging
+* Videoconferencing
+* Advanced phone features (conferences, barging in, voicemail, automation)
+
+So you can do real fancy stuff with it, but I haven't looked at it, because really I just need calls over two households on physical classical wireless handsets.
 
 ## Possible improvements