A while ago I linked to this article – http://martinfowler.com/articles/remote-or-co-located.html – about remote vs co-located teams and which are more effective. I think it still holds up as a good article for guiding thoughts about how to make your team more useful whether co-located or remote or some blend of the two.
Most of the teams I’ve worked with in the last 10+ years have been a mix, with some chunk of the team located at one or two sites and a few more scattered around the world. In those cases, it has helped to get the office-located teams to think about “remote first” modes of communication. Sometimes that becomes “lowest common denominator” communication, where if any one person is on the phone then everyone does the meeting from the phone at their desk. That does help level the field, but when possible it is better to bring everyone up to a higher level with video conferencing or collaborative document editing.
It also helps for the Scrum Master or manager or leads to coach the team about how to communicate with everyone in a way that each team member is not left out or marginalized. That is a great topic of discussion in retrospective meetings.