I'm having a problem with something that used to work brilliantly until a few months ago with Gmail and my Android phone (Sony Xperia Z5 Compact and previously Motorola RAZR M and RAZR i).
Voice messages left on my home phone or my cellphone are recorded either by WorldXChange or by 2talk and automatically copied to my Gmail account. They are then auto-forwarded to one of my email accounts hosted by my ISP. That works, but I am not connected to my email accounts 24/7 and rely on my cellphone telling me I've got a Gmail.
In the past, if I was within cellphone reception range I'd get that notification in near-real-time after the message had been left; if I were out of reception range, I'd get the notification soon (a few minutes) after arriving somewhere with reception. Now, though, the notification is delayed up to 11 hours.
My phone is one of the Xperia Z5 Compacts that Noel Leeming discounted about twenty months ago. It shows some Spark screens while booting up but has never been locked to Spark. (I'm on 2Degrees.) It's running Android 7.0 with Android security patch level 1 March 2017. I get the impression from another thread on this forum that if I'm going to find an Android update it'll require a Spark SIM, as the phone says it's up-to-date whenever I check.
In regards to settings that might affect reception of Gmail notifications:
I've ensured that STAMINA mode is Off. In Settings > Apps > Configure apps > Special access > Battery optimisation I've ticked the box for Gmail to be "NOT OPTIMISED";
I set up a specific Gmail folder to receive and forward the e-mailed voicemail recordings. All that accomplished was to ensure that after however many hours' delay, I get two copies of each voicemail notification when they finally come through.
I'm not wedded to Gmail; indeed, I use it almost exclusively to relay these voicemail messages to myself. If I could set up the same quick-relay functionality that I used to have with Gmail by using an IMAP account on the server that my ISP hosts for me, I'd be happy to do so.
What does everyone suggest? ("Trade the Z5c for an iPhone" is not what I want to hear.)