Has anyone had any luck integrating a call-tracking service with Marketo, or can Marketo do it on its own?
It makes sense to bring them both together if at all possible.
Marketo doesn't do any call tracking OOTB, but if you have call logs that you want to use in the flows or personalize emails based on them, then you could bring them in Marketo (via custom activity/object).
CallRail has a nice little integration with Marketo which creates Custom Activities. You can then use the Marketo REST API to merge people based on Phone Number (or more precisely, a Phone Number mirror field).