@Vittorio.P 

Thank you very much for getting back to me.

Were you trying to day, call transfer via USSD on a different brand router failed as well?

And The error log seemed quite like MR6500V.

As for the first question: I used my phone to run a test.

There is an option “preferred network type” and I could select LTE or 3G.

You could switch to "Perfer 3G" and test Call Transfer via USSD.

@Leoz 

Hey, welcome back, and nice to see you again.

That is awkward now and I am not sure what might be the problem.

And could you please help me run the above test on your smartphone as well to test call transfer over the 3G network?

By the way, when your MR6500V failed to work, did you notice the network type had changed to WCDMA on the system log?

 Or could you please save a copy of system log to support.forum@tp-link.com?