It's theoretically possible Google is accidentally set to block calls from your employer, however the message you hear when that happens is "[three tones] We are sorry. You have reached a number that has been disconnected or is no longer in service." That isn't the message you describe, but I would check Google Voice's History, and also More >> Spam to be sure the calls are not there.
If the calls are not making it to Google Voice, it is possible a routing issue at your employer's landline provider could be causing the problem. In that case, call the landline provider and tell them you can't reach a number with your landline, but you can with your cell phone.
Here's the important part: if they ask who owns the phone number, say it is one of your customers. If they ask who the phone number is with, lie and say you don't know and can't find out. Don't mention anything about VoIP or Google Voice because that will give them an excuse to blame someone else. Just say you can call the number with your cell phone, but not with your landline.
