T46S SfB CAP always uses PSTN

[SebastianM]

Hello all,
we recently configured a T46S SfB in CAP mode (SfB online from Office365 tenant). Of course the corresponding O365 account has the required licenses, too.

But if we now want to call another user from the same tenant, the phone uses only his landline number, not starting a Skype call.
I select the target from the dictionary, and although I see both landline number and e-mail address in the entry, the phone uses only the landline.

And if I delete the landline from the contact and then try to start a call, the phone says "no number" (or similar), although the e-mail address is still valid.

Maybe my expectation is wrong, but I do expect that a Skype phone, regardless of Skype client on PC or desk-phone, always prefers the Skype connected call over the PSTN one.

Can anyone shed some light on this?
Thanks, Sebastian