You might want to grep your syslog to find out why it's hanging up.
Is it server timeout, hangup on inactivity ... ?
You might find, as your persist option was already set
that maxfail was defaulting to 10 and that
10 x (max session time) = ~4 days ???
With maxfail=0, for example, a problem at their end would mean constant
redials (until you're poor