yes, as long as the SME server you are running sail on is not head-of-network. Lets assume you have sail on a server only system with another SME server instance running the network and giving you line of sight to the internet. You can reconfigure the SAIL platform to be server-gateway and have it connect to the second ADSL circuit. Now the phones will use that circuit while everyone else will use the original.
Don't know if I explained that very well but its something we do a lot because unless the customer has fibre, we generally recommend the phones have their own ADSL circuit.
Kind Regards
S