Yealink SIP-T22P VOIP phones unreachable from Asterisk PBX

Environment
- Trixbox 2.6 PBX (Asterisk 1.4)
- Mixture of Linksys SPA 942 and Yealink SIP-T22P VOIP phones
- Phones connecting to the PBX from a remote site over WAN.
Issue
People using Yealink SIP-T22P SIP phones occasionally can't receive incoming calls. Outgoing calls working fine. Linksys phones are not affected. When the issue occurs Asterisk CLI shows phones as UNREACHABLE. Yealink web GUI displays phones as Registered. Rebooting phone resolves the issue until it happens next time.
Worst thing is that users don't have any indication when their phones are not able to receive incoming calls.
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