If you are unable to answer the phone because it is too far away, or because you can't leave the counter then a solution is to install a cordless phone.
Any cordless phone will work, however you might want to think about how you charge the phone up.
If you have a single cordless phone the base will need to be connected to the phone line, so you will need to replace the handset on the base station every night to recharge it.
Depending on the phone you may be able to purchase another way of charging it, either another base or a separate power cable.
However it is often cheaper and easier to purchase the phones as a pack of two.
If you purchase the phone as a dual pack, they you can connect one to the phone, and have the other near the counter.
This is often cheaper and easier than purchasing a separate base for the phone.