hacking calls

[gmaio]

Hi everyone,
I have the same problem of SPAM IP calls coming from ghost numbers as 82,83, 1001, etc.. on only one phone in the company. I know that could sound a bit strange but I have 15 Yealink SIP-T32G in our offices and only one is affected by these spamming calls. I've already updated just yesterday its firmware to ver 32.70.1.33 and its hardware version is reported to be 22.3.2.32.0.0.0. I've already changed the server SIP port and the Outbound proxy server port to 5070. The local SIP port is 5060.
I would like to configure the affected phone (and the others as well) in order to accept only calls coming from the PBX SERVER which is external and Asterisk based. How can I properly configure my SIP-T32G phones to eliminate these spam calls? I've already tried to follow some tutorials I've found in this forum and also on other web sites but they are all intended for different phone models.
Moreover, the company voip telephone network use a dedicated modem/router Cisco Linksys X3000 and a dedicated ADSL. Is there a way to filter the ghost calls at router level in order to protect all the company voip network?
Thanks in advance for any advice on the matter.