And to be clear-
When you said you can connect to it externally, what did you used to connect to it- MySQL client or ?? But you can't connect via MyODBC? (even outside of Access environment, too?)
I'm not quite sure how 'DHCP come from the phone system'... Normally, the router is a DHCP server as well. Is that the case with your network setup?
Does the MySQL box have a hostname?