Firewall and LAN modem

I have a client that I installed ZoneAlarm for. They are on a LAN modem that is always on even though it is a dialup. It worked fine for 2 months then they indicated they could not connect to the net and they had a tech come out and the client said that they first replaced the modem then when that did not work they shut down the firewall and all was well. Now they are upset that I installed a firewall and want reimbursed. My question is – if it worked for months prior why would it be the culprit now and should I suggest that the firewall remain. Though they are on dialup now they plan to install broadband soon.

Questions? – e-mail me