VOIP phones cannot call from extension to extension

[CyberKen1]

Hello Mates,

I have been having an issue for the past few weeks regarding our VOIP phones here in the office. When attempting to dial another employees extension the call goes directly to voicemail but once we attempt to call out to a phone outside of the network, its connects. I had a checkpoint technician work alongside me on this issue and changing the services protocol on one of the rules, which I will leave a screenshot below, seemed to work for a few days and reverted back to the same behaviors. Anybody have any experience with an issue like this?

[Chris_Atkinson]

Which vendor phone system is involved and how are the advanced settings for those services configured?

sk95369 / sk65072 / sk14587 are some excellent VoIP resources for reference.

[CyberKen1]

Thank you for those SKs. I'm sure it'll be very valuable for in the future. CP Tac was able to help me sort out the issue by reconfiguring a rule we had in place already. 

[G_W_Albrecht]

If CP TAC is involved already why not resolve this together with them ? Hard to explain all config here...