Mobile phone reception on Coll

EE operate from all three masts.
Vodafone and O2 only operate from the mast on the hill in the middle of Coll (Cnocan na Ban).

EE's masts have battery and diesel/LPG generator backup. Vodafone and O2 only have short-life battery backup.

However, there's a lot more to this complicated story. Most mobile reception on the east coast of Coll, from Sorisdale down the wilderness coast to Arinagour village, beyond to Hyne and even Port na Luing, receive mobile signals from Ardnamurchan, Mull/Ross of Mull - not as many would like to believe, from Coll's community-owned mast at Cnocan na Ban. Within Arinagour (O2 & Vodafone), the higher positioned houses receive from Cnocan na Ban while the lochside properties and down to Calmac's pier get their signals from Mull. EE's coverage of Arinagour is total as they transmit from the village mast at the BT station.
From Acha, south-west to the airport and around the west coast and as far north as Windy Gap is served by Tiree and/or Coll's Cnocan na Ban mast.
Unfortunately, there are still places without a signal, or it's too week, particularly on beaches and in hollows.

On the face of it, it may appear EE offers the best reception option. However, their Arinagour and Windy Gap masts only transmit on 1800 MHz, frequencies which may only be available to EE contract phones - and not on Pay-As-You-Go deals (this may have changed by now) or other phone suppliers that piggy-back off EE towers (see MVNOs link).
Coll's central Cnocan na Ban mast transmits on 800MHz for all three operators and, additionally, 1800MHz for EE.

Other complications to throw into the mix are that you don't always connect to your nearest mast. However, on Coll this is unlikely as there's little choice and it's hard to image any mast in the inner Hebrides being so swamped with traffic you're re-routed to the next mast.
A likely variable affecting how good your signal is, or where you connect to, is invisible to the user. All the 4G providers reduce their masts' transmission power to the minimum they can get away with. From Huawei data, a typical 4G mast consumes 6-7kW of electricity. If the provider can lower the power of transmissions then their electricity bill will be significantly reduced. Even though Coll's masts are atypically smaller (less powerful), the energy consumed by 4G masts on Coll should concern environmentalists as they, most probably, are the largest consumers of island electricity.
5G will hike up energy consumption by a further 70%. Even scarier, the carbon pollution of energy consumed to support world mobile networks & servers storing our social-media gossip, is greater than that generated by the whole world's air travel.