Yes, I think so.
Go here:
1) Voice Services->Gateways and Trunk Groups->Trunk Group1->TrunkList
change it to any one of these setups you choose to use. I'm not certain if the Trunk can have 3 strunks. I think it should. Two trunks should work.
li,sp1,sp2
li,sp1
li,sp2
2) Then goto
System Management->Outbound Settings->Phone PrimaryLine (and the Attendant PrimaryLine, if you like)
Select the Phone PrimaryLine as Trunk Group 1
This will allow you to make a call on any available line. You may have to tweak your digimaps not to allow any long distance calls going out you POTS line.