T31P & T31G

[Alain-Cayman]

Been using/configuring Yealink IP phones for years throughout our company, mainly T22P and T23P, etc. and we love them, especially ease of configuring local and remote sets.
Have just purchased 3 new T31P and T31G but have a problem I just don't understand. Phones register as remote sets, make/receive calls, and then for no obvious reason, they then cannot be called and cannot make calls, whether internal extension calls or external calls. Seem to still be registered. An hour or two later, they'll work again! Same problem with all three. Did a factory reset, updated firmware, configured exactly as our other Yealink phones, etc., and I see no configuration differences or different fields. Have tried with power supply instead of PoE, etc., but no difference. Why would these phones work only sometimes? CAN ANYONE HELP, PLEASE?
Thanks!
Alain
Grand Cayman