I think QBZappy meant to say in the last line:
QuoteIf most calls are going out SP2 consider doing this: Phone PrimaryLine = SP2
I assume (and first part of QBZappy's advice assumes) that your
current primary line is SP1. That's why you need to force the call over SP2 through **2. If you make SP2 your primary line, you will need to dial **1+number if you want to force calls via SP1. Hopefully, this wasn't too confusing
Finally, **2 and **1 are defaults, but configurable. I changed my Dad's device to use just * to force non-default line.