Three members of staff have tested positive for Covid-19 and a further 16 are now self-isolating as a precaution.
The trust is now investigating the circumstances.
A spokesman for West Midlands Ambulance Service said: "The trust is aware of a social break that a group of staff took last week which has resulted in three testing positive for Covid-19 with a further 16 now self isolating as a precaution.
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"All national guidelines regarding testing and isolating staff have been followed and action was taken as soon as the trust became aware.
"The trust is investigating the circumstances under the disciplinary policy.
"West Midlands Ambulance Service expects the highest standards from its staff both in and out of work. As a healthcare organisation, we are more aware than most of the devastating impact coronavirus has had on the people of the West Midlands and expects staff to fully comply with all of the legislation currently in force.
"The trust continues to respond normally to 999 calls despite this incident."