Not sure why it should make any difference in the 1st place, but try turning your firewall off temporarily. Include with this turning off any spyware, adware & AV programs. Then try telnetting again to your device.
BTW I have heard of different firmwares having different ip's but that is extremely rare.
You can check their site for your particular model & look for (a) a manual, & (b) any firmware upgrades, & (c) telnet info./problems
http://www.zyxel.com/web/product_fam...2-706EE587DAB1 ___ / ___
http://www.zyxel.com/web/support_dow...20040909133019