Based on your description, is it possible that you have a device such as a fax or modem that is trying to call an outside number starting with 7 (page code) without first accessing an outside line? That would explain all the DTMF and the re-tries.
By the way, if you do have faxes on the system you should set their extensions as house phones, set to dial 8 or 92XXX for outside access when they go off hook.
One more possibility is a VM mailbox set for remote notification but number not programmed as CO. If that's the case the mbox user would eventually get a message in mailbox telling him remote notification failed.