When you port your fax number to SRFax, the number is transferred to SRFax and once it has completed, the line connected to your fax machine will no longer work. All calls will be routed directly to SRFax and you will be able to receive multiple calls simultaneously. Also, you will only have to pay SRFax for your fax line.
When you call forward your fax number to your SRFax number, the number remains working with your current provider but it is programmed to forward all incoming calls to the SRFax number. This means that you are able to send a fax from your fax machine that is connected to the line. There are only two down sides to doing this. One is that you are paying your current provider for the line and you are paying SRFax for a line too. Two is that you may not be able to receive multiple calls simultaneously as the call forwarding only forwards one call at a time and the second call will receive a busy signal. Sometimes this is unavoidable if your current number is not portable to SRFax.