Sure, Amoun.
I’m a subscriber of the French provider Free. I have a FP3+ running the default Android 10 distribution updated a few weeks ago (2021-02-05). After reading the above posts I did a quick search and came up with a “how-to” page, cited in my previous post, not published on the official support website of Free, but on that of an independent users’ organisation.
To send the code strings I used the default Phone app, composing the strings as though they were telephone numbers, and using the green “call” button to send.
The following code string functioned correctly to verify the current status of the service : *#61#
I get a pop-up reply on the screen :
“Call forwarding
Voice: +33695600011 after 30 seconds”
Voice presumably refers to the Bearer Service (see the USSD codes Wikipedia page).
The following code string allowed me to set the call forwarding delay to 30s. As I noted above, this is the maximum allowed by Free (according to the users’ tutorial):
**61*+33695600011**30#
- where +33695600011 is the number of the Free voicemail service. This is the number my phone calls when I pick up my voicemail.
Each user needs to be sure of using the correct number as advertised by the provider (or check your phone’s configuration). If your provider uses a shortened number like 555 or 121 that should work.
As I understand the syntax,
-
**61*is an instruction to activate the call forwarding service - call divert when not answered - the phone number is the number you want the calls diverted to
-
**30#indicates no value for the Bearer Service and then the delay (the No Reply Condition Timer) and ends the string.
This seems to conform to the USSD codes as set out in the Wikipedia page indicated by AnotherElk. Remember that the Bearer Service (BS) can be left blank, which gives you the two asterisks after the diversion destination number, but those two asterisks must be there to conform to the syntax.
Finally, as I said in an earlier post, this is old technology, that typically requires strict conformity with syntax rules. There’s plenty of opportunity for error, which would simply result in the request being ignored.