can an employer tell a random person over the phone if an employee is on maternity leave?

Yes, maternity is not an illness.

I thought it was contagious, since you catch it from someone, It may well be against company rules to give employees persoal info out.

In truth probably not but we all do it.

An employees details are strictly confidential, I don't think this statement would come under this title.. a bit like saying... it's their day off