Whatever works.
I know a Mitel dealer in a resort area whose currently upgrading scores of end-of-life sx20s to ICPs, and he's using IP trunking for ROI (people used to use Call accounting for ROI). The 20s are starting to wear out, so it's a good time for it.
In 5 years, it'll be the 50s, and in 5-7 years, it'll be end-of-life on sx100/200 Analog, imho.
But if you're happy and the customer's happy, more power to you, and carry on.
