Right now, this is not yet possible.

This is what our Collanos Phone specialist wrote:
We use Jabber as our Collanos-Chat-Protocol. At the moment each Jabber-Contact is identified as Collanos-Contact. Basic things like "can I phone this user?" and Address-Extension (add @collanos.net if someone only enters the nick) are handled by checking whether this is a jabber account/contact or not.
The phone has its own Mapping to the GaIM-Accounts through an IMWrapper. So the easiest would be to add a new protocol (another wrapper around the jabber-protocol) which we use for not-collanos-jabber-accounts and contacts. [...]
Regards, Pirmin
I added this as a high-priority requirement, i.e. to support any XMPP based contacts, for the next Collanos Phone release. We hope to be able to roll this feature out later this year.