This is in the listener.ora file in one of my databases. You may want to use the Startup wait and Connect timeout parameters in the listener.ora file on you db server. It could be that you connection is slow and it takes to long for the client to connect. You might also try sqlnet tracing. Also make sure try to hard ccode the ip and make sure that tcp/ip is selected for the dial up connection.

LISTENER =
(ADDRESS_LIST =
(ADDRESS =
(PROTOCOL = TCP)
(HOST = 192.168.0.1)
(PORT = 1521)
)
)
STARTUP_WAIT_TIME_LISTENER = 0
CONNECT_TIMEOUT_LISTENER = 0
TRACE_LEVEL_LISTENER = OFF
SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(SID_NAME = BSG1)
(ORACLE_HOME = \Oracle\ora81)
(GLOBAL_DBNAME = MYSID)
)
)