V90 56K fax modem connecting at 28.8K

I was using a 28.8Kbps fax modem for dialing into the internet. My maximum connection speed was around 33kbps. Now I installed a V90 56KBPS PCI data fax modem card and I can only connect at 28.8max. I am still using the same physical cable line.
How can optimize this modem