According to Wikipedia, the sole landline and mobile phone operator in Eritrea is the Eritrean Telecommunications Services Corporation, more commonly known as EriTel.

Being no expert in mobile triangulation myself, I’m thinking that the Libyan phone operators would be more helpful as the passengers were in Libya when they boarded the Ghost Boat. There seems to be two operators: Al Madar and Libyana.

As you realise, it’s a long shot. With their assistance, even reverse phone lookups could be better performed.