Landline calls don't always connect

We have Comcast business and internet and have noticed a problem (at least intermittently) with calls getting through. I have had people say they called the store and the phone rang and rang, and no one picked up. This is when I was right here and the phone on my desk never rang. This has happened with co-workers (who then call my cell phone) and also with customers. Invariably, when I ask, they say they called from a landline. If they try again calling from a cell phone, it connects with no problem.

I've called Comcast support, they called me from a landline and of course it connected without any trouble. ::eye roll::

Has anyone else had an experience like this? Does it have to do with what phone service the caller has, or what could be causing it? I worry that it's making us lose business...not at all what we were hoping for when choosing this carrier. 

Appreciate any input - Elizabeth