Forty-five migrants were taken by ferry from Lesbos to Turkey as Greek authorities restarted returns under an EU deal to stem the flow of migrants and refugees to Europe.
The ferry left the port of Mytilene on Friday morning, with a second boat expected to leave later, state television reported.
The first group of 202 migrants to be returned, most of them from Pakistan and Afghanistan, were taken back to Turkey on Monday.
The Prime Minister has offered to provide more help to Greece as it struggles to return migrants to Turkey.
The EU-Turkey agreement to tackle the migrant crisis took effect on Monday but Greek authorities have warned the operation is threatened by a shortage of personnel.
A Downing Street spokesman said Mr Cameron spoke to Mr Tsipras on the phone on Tuesday afternoon.
The Democratic presidential candidate may also have shown his cards on his choice of running mate.
The US president also shared a post on Twitter accusing Dr Anthony Fauci of misleading the public over hydroxychloroquine.
Fears over an impending second wave of coronavirus dominates Wednesday’s front pages.