I have a fax line on a VG248. User replaced Brother model(which worked fine) with an HP C6150. The new fax machine can send faxes, but not receive. The machine rings once, I hear what sounds like negotiation beginning, but the fax machine keeps ringing. I tried the machine on a Centrex line and it works fine. I put it back on the VG248 and it quits working. Any ideas?