Exch 5.5 IMC stays off-hook

I run MS Small Business Server 4.5 (5 clients) which has a cut down Exchange 5.5 Server as a component.
The Exchange IMC is set to dial out to our ISP every 2 hours during daytime or when there is outgoing traffic. It is set to disconnect after 5 minutes. This normally works. Occasionally the IMC simply doesn’t disconnect and the server stays online until someone clears it manually. This could be over a long weekend! Mail can be delivered all through this period. Any suggestions why and how to stop it.
We also use Proxy Server and the logs indicate no interference though Proxy Server will operate while the mail is online.
Present possible solutions are to disconnect ‘phone line at night and change ISP/line-charging scheme. (UK systems)