If you are experiencing one way voice particularly on incoming calls the most probable cause is SIP ALG. While originally designed to resolve a NAT related problems, with no standard implementation many routers and software based firewalls corrupt the SIP message by attempting to rewrite part of the SIP message. Frustratingly the one-way voice issues will often only occur on a single number and will clear once the router has been reset.

See also Disabling SIP ALG.

If you are unable to access the router consider switching your phone or PBX to use TLS which to encrypt the SIP packet ultimately preventing SIP ALG corruption (see TLS).

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