I've never considered using syntax like that before, but it works. You do have to concatenate '%' + interfaces.CircuitID + '%' though.
It would seem more obvious than my choice, but I do a TON of substringing and text manipulation in my scripts and procs so Patindex is a common occurrence and I'm usually testing for 0 cases prior to using it as a starting point for a substring.
If you're searching on a non-indexed field Patindex is faster.