Outside calls ring on intercom

[haironfire]

We have an R7 installed with 5 lines. 3 lines appear(immR) on all exts. 2 lines appear(immR) on 2 exts only.
Intermittently ext 11 will have an outside call ring on intercom, she answers and talks to caller. We have verified the call has not been transferred but have not yet verified what number the caller dialed.
No auto-attendant but all have PVM mailboxes.
Have never seen this before. Any ideas?

 

[haironfire]

Oops! Customer only has 3 lines, appear on all sets. She is verifying dialtone and features with provider now.

 

[haironfire]

Got it, there was another line. Was answered by AA but only appeared on 1 ext. Dial 0 calls transferred to ext without line appearance.