I'm also facing the same issue with my Note10+.
I've tried the followings:
1. Trigger VoLTE off n on
2. Restart the phone
3. Soft reset (adv by Samsung)
4. Reset network (adv by Samsung)
5. Reset APN (adv by Samsung)
6. Clear cache (adv by Samsung)
7. Change SIM card (adv by Singtel)
8. Reset my line by Singtel (adv by Singtel)
9. Insert other SIM cards.
VoLTE was working fine in my previous phones i.e. Note8 n S7 Edge. Until i changed to Note10+ I realised VoLTE not triggered. I called Singtel to check n they said everything seems ok. I went to singtelshop n they suggested could be the SIM issue n have it replaced. I tried putting 2 other Singtel SIMs n VoLTE showing. When my SIM inserted into 2 other phones, VoLTE dun show as well. Went to Samsung n they did a check, all ok.
Wasted more than 100mins talk time calling 1688! Sigh, still not resolved. All they said was from the system everything in order, 4G clearvoice, 4G VAS etc. all provided... line also reset... Keep saying 4G LTE n VoLTE are the same...
So i think it narrowed down to 3 possibilities: 1. My line (already reset twice), 2. My SIM card (already replaced) 3. My mobile plan. I previously using EasiMobile plan (discontinued) and changed to XO plan while the 2 SIM cards I tried are under Combo plans. Dunno about the plans on the SIMs Samsung tried.
Could it be XO plan do not have VoLTE?
Anyone under XO plan using Note10+ facing the same issue?