LTE smartwatches may get up to 90% of the communications range of a smartphone. Most providers have turned off (or are turning off) 2G and 3G so coverage may be dynamic. Generally Bluetooth headsets can be used with LTE smartwatches, which helps call quality for any phone device.

Mobile devices including smartwatches may switch frequency bands when going from idle to phone call or data usage:

E

2G EDGE, the oldest digital network mode still in use, very slow.

H

3G HSPA/HSPA+, good enough for basic web browsing and email.

4G

really good 3G. Carriers may call their upgraded 3G networks 4G.

LTE

actually using 4G LTE.

5G

not necessarily faster than LTE, but could be

The signal bars may jump up/down a few notches when going from idle to active due to the phone band switching e.g. 700 MHz vs. 1900 MHz. Apps like Network Cell Info can help reveal these behaviors.