I was testing today Suspend to Ram on Librem 5 it work really good, the modem woke up from deep sleep on 3 tones of the call.
It seems that Modem wake up with 4G only, but at moment there is issues with call under 4G for missing VoLTE so to fix this issues with STR we need to put the modem on all Signal on Phosh config. Like 2G-3G-4G.
I am pretty sure that oFono 1.33 has a hack for permit call under 4G without going back to 3G, so L5 it using MM.
Thank you Purism!