I've had numerous incoming calls "drop" while my physical phone only rings one or two times.
When I check the Expert Config there are red exclamation points present on a few lines.
See attachments
Thanks for looking.
The red exclamation marks mean that those settings have been changed from their default values. Quite normal and I see nothing untoward with their values now which would give your call drops.
Looks like you are using Callcentric.