With my Aastra 6867i approximately 50% of the time I begin to dial a 10 or 11 digit number with the phone on hook the number disappears part way through dialing. I almost get the impression some type of BLF or screen update is wiping it.
Using FreePBX Distro 6.12.65-31 w/ EPM, firmware v 4.1.0.128 on the phone (EPM Aastra firmware package 1.11)
There’s a bug in Aastra’s firmware that causes the dialed number to clear whenever a registration renews. I reported it to them more than a year ago, and am surprised to hear that it is not fixed. Are you using the current firmware?
This but does not affect 67xx phones.
I have a 6869i on my desk and it does the same thing - not as often - I thought it was a timeout.
If you change your registration timer to a longer period, it should happen less often.