DTMF for carrier pre-selection

[rpereira]

I apologise for using the wrong terminology but not sure how exactly to search for this problem:
I bought a T21P E2, which I have connected to a fritz.box 5490 together with a few more DECT phones.
On the DECT phones if I call "*121#" this allows me do VOIP provider pre-selection on the fritz.box and receive the dial tone to then being able to enter the number going out through that provider.
I'm trying to use that in the IP phone (ultimately to associate with a DSS key), but can't get it working.
When I send "*121#", I get immediately "user busy" and it seems to me as if that particular code is sent straight-through to the VOIP provider, since if I send "*121#1234566" it works as expected calling 1234566 using the selected provider.
Does anyone know what settings would allow me to have the DSS key working to do this pre-selection, and receive the dial tone to enter phone number as a second step ?

Thanks