Phone tennis is usually unintentional and frustrating. Though in exceptional circumstances could be used as a stalling tactic.
Most common where
1) either or both callers do not have a mobile phone
2) either or both mobiles are often switched off or unheard
3) either or both phones are often engaged
4) both caller have busy schedules
5) both callers know they are meant to have a difficult conversation but would prefer to avoid it
To diffuse each other's annoyances when you finally make contact after receiving their nth message "Hi, looks like we've been playing phone tennis,..."