If you connect using VFC and line quality drops below that allowable for a 14,400 connection, the modems will disconnect. If this occurs frequently for a particular call, you will want to disable VFC before calling that modem again. These connections are more likely to drop. For these calls, you can force a lower connect speed by locking the modem to a lower link rate. Dropped V.34 Connections and V.34 Rate Switching.
Dropped connections can occur when there is a sharp decrease in line quality during a call.
V.34 modems will switch to rates as low as 4,800 bps to compensate for these changes. If the loss of quality is extremely severe, even V.34 will drop the connection. Technical phone line bandwidth requirements, and how a connection's bandwidth and symbol rates are determined. As already stated, V.34 and VFC connection rates are based on the available bandwidth over the phone line. V.34 and V.FC modulations allow adjusting the symbol rate to any of six possible values, to obtain the best match with the available bandwidth.