Is there any way to block an incoming call (phone number) either in the IP Server 900 or in ESI Hosted Sip Trunking service?

Are you referring to ghost calls

they are refering to an ex that keeps calling most likely .

Contact ESI, they'll block it. The 900 can't route on caller ID. My money's on the ex calling without caller ID or a different originating number once he/she figures out their number is blocked.