callforwarding: initial documentation for self

Resolves: Mikaela/privgist#4

callforwarding/README.md

@@ -0,0 +1,36 @@
+# Call forwarding, relevant commands edition
+
+*This is widely documented around the internet, but everything has a lot
+ of details irrelevant to my case of two phones and it's always effort to
+ cherry-pick the relevant parts. This file may be unintentionally Finland
+ specific.*
+
+1. Open dialer
+2. Enter command. 358 is Finnish international format, remove leading 0 from
+   phone number's local format. (040… → 3584…)
+3. Call there
+
+## Commands
+
+* `##002#` - Remove all call forwards
+* `**61*358…**30#` - forward if call is not answered in 30 seconds
+  * 30 may be replaced with multiples of 5 between 5 and 30.
+* `**62*358…#` - forward if phone is not connected to network.
+* `**67*358…#` - forward if phone is on another call.
+
+Copy-paste template for text editor:
+
+```
+**61*358…**30#
+**62*358…#
+**67*358…#
+```
+
+## GUI
+
+* Apple: has no proper GUI for the cases above, only on/off toggle on forwarding
+  every call. https://support.apple.com/guide/iphone/iphe9bdd027a/ios
+* Android: likely OEM-dependent and I don't have an Android with a SIM.
+* SailfishOS: Settings → Applications → Phone → Call forwarding. Case forward when
+  not answered in X time has no option how long to wait and I am not going to try
+  whether it forwards in 5 or 30 seconds.