Got SIP response 488 "Not Acceptable Here" back fr

When I try to dial in to a line on a Sipura, I get the following messages on the console and the call does now go through.

Got SIP response 488 "Not Acceptable Here" back from x.x.x.x
 SIP/972xxxxxxx-09de8a90 is circuit-busy
  == Everyone is busy/congested at this time (1:0/1/0)