Been using 3G/HSDPA+ and 4G as part of my Internet Access for donkeys years now and the Speeds have always been more positive, consistent and Cost effective over Fixed Line access that have been offered by Who Ever.
As for running a Fixed Phone via VOIP through the Router that can be a bit touch and go, your better off using a Router that will give you a Dumb PSTN Socket on board plug for a normal Phone and run that via the 2G GSM side of it, If, you really must need to run a Fixed Line Phone (with a GSM Number) and hope that the Network Provider will still continue to support 2G Services.
As for , External Aerials, yes a must, you need to get the highest level of Signal Strength or better still, a multiple selection of Cell sites available.
With the 5G now looking at being available in Larger or Unlimited Data Plans, no reason to suggest running a set up like should change, just only improve.
Ive always predicted that with 5G/6G Services being over saturated in Coverage when in the completion phases, the Big 5G/6G Network Player will be offering 4,5 and 6G Tier Fixed Line and Home Broadband Services as a replacement to the Cable from the Street.