Cisco :: Cucme Through Sip Trunk Not Receiving Calls From Landlines

I heard from customers calling into the office stating that they have experienced some issues with our prompts and the extension input, that it is not recognizing their input. Stating sometimes it does not work on their land line but once they call back on their cell phone it works. We in the office all had people from the outside call from a land line to our office to verify the issue but the calls went through. We are thinking that they may be dialing the wrong extensions.Has anyone had any issues with land lines having trouble calling in.I am running cucme connected to a sip trunk.