I have a very old (1947) WE 302 phone connected to an OBi 200. Yes, the sound quality isn't great, compared to modern phones, but it's acceptable. Aside from the good advice from drgeoff...
Try going into OBi expert mode on the OBiTALK portal page for your device. Go to the "Physical interfaces" --> Line 1 section, and find ChannelTxGain and ChannelRxGain. I bumped mine up to -3 and -2 respectively. I have a modest hearing loss, but I can hear comfortably and people can hear me at those settings. You can experiment with higher or lower settings to find what's best for you.