Did you check with your SIP registrar and proxy to make sure the Tenor is registered to it correctly?
Additionally, the SIP software for the Gen 1 Tenors like the A400 and A800, is only Beta. It is not a full tested release from Quintum. What that means is that it should work, but there may be some bugs that they did not see. Also, since Quintum has announced end of life on the gen 1 products, it is doubtful that they will make any further software for them.