I use T-Mobile and vist the Shetlands frequently. There's absolutely zero reception at Sumburgh. As for in Lerwick, I think it's better towards the north.(Co-op onwards) and I get a very good reception at Green head where my ship usually berths. If I'm in the pubs in Lerwick I tend to have to go outside to make a call which is alright in summer but, not so good the rest of the time.