Mobile phone reception on Coll
There are currently (Jan 2020) two operational mobile phone masts on Coll with a third in the pipeline.
EE operate from both masts (soon to be all three).
Vodafone and O2 only operate from the mast on the hill in the middle of Coll.
All three of EE's masts have battery and diesel generator backup. Vodafone and O2 only have short-life battery backup.
Those are the facts presented in an unbiased way, please make of it as you will.
However, that's not the whole story. Most reception on the east coast of Coll, from Sorisdale down the wilderness coast to Arinagour village and beyond to Hyne, receive mobile signals from Mull - not as many believe, from Coll's initial mast in the centre of the island.
From the airport around to the the west coast and as far north as Windy Gap is served by Coll's first mast. Many places also receive from Tiree.
Sadly, there are still many places where there isn't a signal, or it's too week (mainly a Vodafone thing), particularly on beaches and in hollows.
The new mast, about to be erected on Windy Gap, will serve the east end of Coll from Gallanach to Sorisdale but only for those on EE; and, apparently, it's very unlikely any other provider will be allowed to use this mast.
On the face of it, it may appear EE offers the best option. However, their Arinagour mast (and probably their soon-to-be-erected mast at Windy Gap) only transmit on 1800 MHz, frequencies only available to EE contract phones - and not on Pay-As-You-Go deals - and not available to other phone suppliers that piggy-back off EE towers (see MVNOs link).
Coll's first mast, in the island's centre (well almost central) transmits on 800MHz and 1800MHz .
The situation is dynamic with improvements all the time, so please keep an eye on this page. I'll do my best to update as soon as I get any information but real information i.e. not social-media gossip, is sometimes difficult to come by.
MVNOs: a guide to virtual networks (which network your mobile phone works on)