I was thinking more in terms of transmission speed. I sometimes need to send ~15mb photos on deadline, so it's kind of important to me.
If you have an active VoLTE call and the call drops when switching from LTE to 3G, this means that the operator you're using doesn't support SRVCC (
Single Radio Voice Call Continuity). This feature is still not widely supported among network operators, at least in Europe. In such cases, switching the VoLTE support off is the only way to avoid having dropped calls if you don't always have LTE coverage. Without VoLTE, a feature called CSFB (
Circuit Switched FallBack) is used: everytime a call is received or started, the phone first switches to 3G or 2G, then the call is established.
When the call is released, most of the times the phone should register back to LTE: the network can force the phone to do a 2/3G->LTE handover, or it expects it to perform this operation (e.g. all the phones certified for the european Vodafone networks are expected to behave that way). If the network does not implement nor require this, it's up to the phone's manufacturer to decide if the phone should go back to LTE or not.
The drawback of having VoLTE disabled is that any data transmission
might continue after a handover from LTE to 3G/2G (depends again on the operator if it supports this), albeit at a slower speed, but if the data transmission is still ongoing there's no way to return back to LTE. This mean that, when a call is dialled/received, the phone switches to 3G and the data transmission might still continue; once the call is released, the phone will remain in 3G until the transmission ends, and only then it can return back to LTE.
As for LTE vs. 3G downlink/uplink speed differences, theoretically in 3G you can go up to 672 Mbpss downlink/168 uplink (8 carrier MC-HSPA), in LTE 300/75 Mbps (until LTE Advanced will be deployed), actually the real speeds are usually much lower. E.g. in UK, LTE average speeds are between 29 and 15 Mbps (
https://www.4g.co.uk/vodafone-4g-network-summary/#speeds), in 3G between 6.8 and 5.2 (
https://www.ofcom.org.uk/about-ofcom/latest...3g-4g-bb-speeds).
Varti