As Mo has said, there are those who can play and call at the same time but they are few and far between. Both activities are very demanding and not many people are able to do both simultaneously.

More often where band member also call they will begin with calling and only start playing once the dance is in full flow and the dancers need less guidance. This is easier if they are in a supporting musical role rather than playing lead instrument, and where they can revert to calling should the need arise without disruption to the music.

What is important is to agree who will be responsible for arranging the caller, and whether the band's fee includes the caller or they will paid separately. In my (UK) experience, private events usually expect the band to organise this, but dance clubs and folk festivals.often prefer to book a caller of their choice.