Try Phone Calling in Preview portal failing

Hi,

I'm trying out the https://learn.microsoft.com/en-us/azure/communication-services/concepts/telephony/try-phone-calling on ACS in the Preview Azure Portal, and I get an error for even the most basic thing. I set up an ACS resource, and added a phone number for outbound and inbound calls. When I try to make a call, I get an error Call failed SIP code 603 Decline subcode 560603

I must be overlooking something really obvious. Can anybody help ?

Stephane